diff --git a/gen/schema_gen.go b/gen/schema_gen.go
index 263164eb5..8e0ea538a 100644
--- a/gen/schema_gen.go
+++ b/gen/schema_gen.go
@@ -261,12 +261,14 @@ func (g *schemaGen) generate2(name string, schema *jsonschema.Schema) (ret *ir.T
 		hasProps := len(schema.Properties) > 0
 		hasAdditionalProps := false
 		denyAdditionalProps := false
+		hasPatternProps := len(schema.PatternProperties) > 0
+		isPatternSingle := len(schema.PatternProperties) == 1
 		if p := schema.AdditionalProperties; p != nil {
 			hasAdditionalProps = *p
 			denyAdditionalProps = !*p
+		} else if !hasProps && !hasPatternProps {
+			hasAdditionalProps = true
 		}
-		hasPatternProps := len(schema.PatternProperties) > 0
-		isPatternSingle := len(schema.PatternProperties) == 1
 
 		if !hasProps {
 			if (!hasAdditionalProps && isPatternSingle) || (hasAdditionalProps && !hasPatternProps) {