diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index f0d2f5d..692d7dd 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -36,6 +36,7 @@ When releasing a new version:
 ### Bug fixes:
 
 - genqlient now explicitly rejects argument, operation, and type names which are Go keywords, rather than failing with an opaque error.
+- genqlient now gives better error messages if it thinks your schema is invalid.
 
 ## v0.4.0
 
diff --git a/generate/parse.go b/generate/parse.go
index 2f62392..4569f25 100644
--- a/generate/parse.go
+++ b/generate/parse.go
@@ -10,11 +10,10 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/vektah/gqlparser/v2"
 	"github.com/vektah/gqlparser/v2/ast"
-	"github.com/vektah/gqlparser/v2/gqlerror"
 	"github.com/vektah/gqlparser/v2/parser"
 	"github.com/vektah/gqlparser/v2/validator"
+	_ "github.com/vektah/gqlparser/v2/validator/rules"
 )
 
 func getSchema(globs StringList) (*ast.Schema, error) {
@@ -32,21 +31,41 @@ func getSchema(globs StringList) (*ast.Schema, error) {
 		sources[i] = &ast.Source{Name: filename, Input: string(text)}
 	}
 
-	// Multi step schema validation
-	// Step 1 assume schema implicitly declares types that are required by the graphql spec
-	// Step 2 assume schema explicitly declares types that are required by the graphql spec
-	var (
-		schema       *ast.Schema
-		graphqlError *gqlerror.Error
-	)
-	schema, graphqlError = gqlparser.LoadSchema(sources...)
+	// Ideally here we'd just call gqlparser.LoadSchema. But the schema we are
+	// given may or may not contain the builtin types String, Int, etc. (The
+	// spec says it shouldn't, but introspection will return those types, and
+	// some introspection-to-SDL tools aren't smart enough to remove them.) So
+	// we inline LoadSchema and insert some checks.
+	document, graphqlError := parser.ParseSchemas(sources...)
 	if graphqlError != nil {
-		schema, graphqlError = validator.LoadSchema(sources...)
-		if graphqlError != nil {
-			return nil, errorf(nil, "invalid schema: %v", graphqlError)
+		// Schema doesn't even parse.
+		return nil, errorf(nil, "invalid schema: %v", graphqlError)
+	}
+
+	// Check if we have a builtin type. (String is an arbitrary choice.)
+	hasBuiltins := false
+	for _, def := range document.Definitions {
+		if def.Name == "String" {
+			hasBuiltins = true
+			break
 		}
 	}
 
+	if !hasBuiltins {
+		// modified from parser.ParseSchemas
+		var preludeAST *ast.SchemaDocument
+		preludeAST, graphqlError = parser.ParseSchema(validator.Prelude)
+		if graphqlError != nil {
+			return nil, errorf(nil, "invalid prelude (probably a gqlparser bug): %v", graphqlError)
+		}
+		document.Merge(preludeAST)
+	}
+
+	schema, graphqlError := validator.ValidateSchemaDocument(document)
+	if graphqlError != nil {
+		return nil, errorf(nil, "invalid schema: %v", graphqlError)
+	}
+
 	return schema, nil
 }
 
diff --git a/generate/testdata/errors/InvalidSchema.go b/generate/testdata/errors/InvalidSchemaSyntax.go
similarity index 100%
rename from generate/testdata/errors/InvalidSchema.go
rename to generate/testdata/errors/InvalidSchemaSyntax.go
diff --git a/generate/testdata/errors/InvalidSchema.graphql b/generate/testdata/errors/InvalidSchemaSyntax.graphql
similarity index 100%
rename from generate/testdata/errors/InvalidSchema.graphql
rename to generate/testdata/errors/InvalidSchemaSyntax.graphql
diff --git a/generate/testdata/errors/InvalidSchema.schema.graphql b/generate/testdata/errors/InvalidSchemaSyntax.schema.graphql
similarity index 100%
rename from generate/testdata/errors/InvalidSchema.schema.graphql
rename to generate/testdata/errors/InvalidSchemaSyntax.schema.graphql
diff --git a/generate/testdata/errors/InvalidSchemaWithBuiltins.graphql b/generate/testdata/errors/InvalidSchemaWithBuiltins.graphql
new file mode 100644
index 0000000..36cf9f0
--- /dev/null
+++ b/generate/testdata/errors/InvalidSchemaWithBuiltins.graphql
@@ -0,0 +1 @@
+query MyQuery { f g }
diff --git a/generate/testdata/errors/InvalidSchemaWithBuiltins.schema.graphql b/generate/testdata/errors/InvalidSchemaWithBuiltins.schema.graphql
new file mode 100644
index 0000000..297c238
--- /dev/null
+++ b/generate/testdata/errors/InvalidSchemaWithBuiltins.schema.graphql
@@ -0,0 +1,101 @@
+type Query {
+  f: String
+  g: Bogus
+}
+
+scalar Int
+scalar Float
+scalar String
+scalar Boolean
+scalar ID
+
+directive @include(if: Boolean!) on FIELD | FRAGMENT_SPREAD | INLINE_FRAGMENT
+directive @skip(if: Boolean!) on FIELD | FRAGMENT_SPREAD | INLINE_FRAGMENT
+directive @deprecated(reason: String = "No longer supported") on FIELD_DEFINITION | ARGUMENT_DEFINITION | INPUT_FIELD_DEFINITION | ENUM_VALUE
+directive @specifiedBy(url: String!) on SCALAR
+
+type __Schema {
+  description: String
+  types: [__Type!]!
+  queryType: __Type!
+  mutationType: __Type
+  subscriptionType: __Type
+  directives: [__Directive!]!
+}
+
+type __Type {
+  kind: __TypeKind!
+  name: String
+  description: String
+  fields(includeDeprecated: Boolean = false): [__Field!]
+  interfaces: [__Type!]
+  possibleTypes: [__Type!]
+  enumValues(includeDeprecated: Boolean = false): [__EnumValue!]
+  inputFields: [__InputValue!]
+  ofType: __Type
+  specifiedByURL: String
+}
+
+type __Field {
+  name: String!
+  description: String
+  args: [__InputValue!]!
+  type: __Type!
+  isDeprecated: Boolean!
+  deprecationReason: String
+}
+
+type __InputValue {
+  name: String!
+  description: String
+  type: __Type!
+  defaultValue: String
+}
+
+type __EnumValue {
+  name: String!
+  description: String
+  isDeprecated: Boolean!
+  deprecationReason: String
+}
+
+enum __TypeKind {
+  SCALAR
+  OBJECT
+  INTERFACE
+  UNION
+  ENUM
+  INPUT_OBJECT
+  LIST
+  NON_NULL
+}
+
+type __Directive {
+  name: String!
+  description: String
+  locations: [__DirectiveLocation!]!
+  args: [__InputValue!]!
+  isRepeatable: Boolean!
+}
+
+enum __DirectiveLocation {
+  QUERY
+  MUTATION
+  SUBSCRIPTION
+  FIELD
+  FRAGMENT_DEFINITION
+  FRAGMENT_SPREAD
+  INLINE_FRAGMENT
+  VARIABLE_DEFINITION
+  SCHEMA
+  SCALAR
+  OBJECT
+  FIELD_DEFINITION
+  ARGUMENT_DEFINITION
+  INTERFACE
+  UNION
+  ENUM
+  ENUM_VALUE
+  INPUT_OBJECT
+  INPUT_FIELD_DEFINITION
+}
diff --git a/generate/testdata/errors/InvalidSchemaWithoutBuiltins.graphql b/generate/testdata/errors/InvalidSchemaWithoutBuiltins.graphql
new file mode 100644
index 0000000..36cf9f0
--- /dev/null
+++ b/generate/testdata/errors/InvalidSchemaWithoutBuiltins.graphql
@@ -0,0 +1 @@
+query MyQuery { f g }
diff --git a/generate/testdata/errors/InvalidSchemaWithoutBuiltins.schema.graphql b/generate/testdata/errors/InvalidSchemaWithoutBuiltins.schema.graphql
new file mode 100644
index 0000000..e81082f
--- /dev/null
+++ b/generate/testdata/errors/InvalidSchemaWithoutBuiltins.schema.graphql
@@ -0,0 +1,4 @@
+type Query {
+  f: String
+  g: Bogus
+}
diff --git a/generate/testdata/snapshots/TestGenerateErrors-InvalidSchema-go b/generate/testdata/snapshots/TestGenerateErrors-InvalidSchema-go
deleted file mode 100644
index 6c9eca4..0000000
--- a/generate/testdata/snapshots/TestGenerateErrors-InvalidSchema-go
+++ /dev/null
@@ -1 +0,0 @@
-testdata/errors/InvalidSchema.schema.graphql:4: invalid schema: Expected :, found }
diff --git a/generate/testdata/snapshots/TestGenerateErrors-InvalidSchema-graphql b/generate/testdata/snapshots/TestGenerateErrors-InvalidSchema-graphql
deleted file mode 100644
index 6c9eca4..0000000
--- a/generate/testdata/snapshots/TestGenerateErrors-InvalidSchema-graphql
+++ /dev/null
@@ -1 +0,0 @@
-testdata/errors/InvalidSchema.schema.graphql:4: invalid schema: Expected :, found }
diff --git a/generate/testdata/snapshots/TestGenerateErrors-InvalidSchemaSyntax-go b/generate/testdata/snapshots/TestGenerateErrors-InvalidSchemaSyntax-go
new file mode 100644
index 0000000..0120cee
--- /dev/null
+++ b/generate/testdata/snapshots/TestGenerateErrors-InvalidSchemaSyntax-go
@@ -0,0 +1 @@
+testdata/errors/InvalidSchemaSyntax.schema.graphql:4: invalid schema: Expected :, found }
diff --git a/generate/testdata/snapshots/TestGenerateErrors-InvalidSchemaSyntax-graphql b/generate/testdata/snapshots/TestGenerateErrors-InvalidSchemaSyntax-graphql
new file mode 100644
index 0000000..0120cee
--- /dev/null
+++ b/generate/testdata/snapshots/TestGenerateErrors-InvalidSchemaSyntax-graphql
@@ -0,0 +1 @@
+testdata/errors/InvalidSchemaSyntax.schema.graphql:4: invalid schema: Expected :, found }
diff --git a/generate/testdata/snapshots/TestGenerateErrors-InvalidSchemaWithBuiltins-graphql b/generate/testdata/snapshots/TestGenerateErrors-InvalidSchemaWithBuiltins-graphql
new file mode 100644
index 0000000..1c33e00
--- /dev/null
+++ b/generate/testdata/snapshots/TestGenerateErrors-InvalidSchemaWithBuiltins-graphql
@@ -0,0 +1 @@
+testdata/errors/InvalidSchemaWithBuiltins.schema.graphql:3: invalid schema: Undefined type Bogus.
diff --git a/generate/testdata/snapshots/TestGenerateErrors-InvalidSchemaWithoutBuiltins-graphql b/generate/testdata/snapshots/TestGenerateErrors-InvalidSchemaWithoutBuiltins-graphql
new file mode 100644
index 0000000..3c07e62
--- /dev/null
+++ b/generate/testdata/snapshots/TestGenerateErrors-InvalidSchemaWithoutBuiltins-graphql
@@ -0,0 +1 @@
+testdata/errors/InvalidSchemaWithoutBuiltins.schema.graphql:3: invalid schema: Undefined type Bogus.