diff --git a/src/ServiceStack.Api.Swagger/SwaggerApiService.cs b/src/ServiceStack.Api.Swagger/SwaggerApiService.cs
index e943c30635a..d0a76cbb708 100644
--- a/src/ServiceStack.Api.Swagger/SwaggerApiService.cs
+++ b/src/ServiceStack.Api.Swagger/SwaggerApiService.cs
@@ -524,7 +524,7 @@ private List<MethodOperationParameter> ParseParameters(string verb, Type operati
                     where !string.Equals(member.ParameterType, "model") 
                     select new MethodOperationParameter
                     {
-                        DataType = member.DataType,
+                        DataType = member.DataType ?? SwaggerType.String,
                         AllowMultiple = member.AllowMultiple,
                         Description = member.Description,
                         Name = member.Name ?? key,