diff --git a/Upsilon/Binder/Binder.cs b/Upsilon/Binder/Binder.cs
index 5b29008..3f80d73 100644
--- a/Upsilon/Binder/Binder.cs
+++ b/Upsilon/Binder/Binder.cs
@@ -292,6 +292,14 @@ namespace Upsilon.Binder
                     if (udSymbol.Parent.Properties.TryGetValue(resolved.Name, out var ubProperty) && ubProperty is UserDataBoundMethod ubMethod)
                     {
                         returnType = ubMethod.ResultType;
+                        var (isValid, error, wrongParameter) = ubMethod.ValidateParameters(parameters.ToImmutable());
+                        if (!isValid)
+                        {
+                            var span = e.Span;
+                            if (wrongParameter != null)
+                                span = wrongParameter.Span;
+                            _diagnostics.LogError(error, span);
+                        }
                     }
                 }
             }
diff --git a/Upsilon/BoundTypes/UserDataBoundTypeDefinition.cs b/Upsilon/BoundTypes/UserDataBoundTypeDefinition.cs
index 1af8849..59f1770 100644
--- a/Upsilon/BoundTypes/UserDataBoundTypeDefinition.cs
+++ b/Upsilon/BoundTypes/UserDataBoundTypeDefinition.cs
@@ -1,8 +1,10 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.Linq;
 using System.Reflection;
 using Upsilon.BaseTypes;
+using Upsilon.Binder;
 using Upsilon.StandardLibraries;
 using Type = Upsilon.BaseTypes.Type;
 
@@ -67,11 +69,20 @@ namespace Upsilon.BoundTypes
                 {
                     methodName = sfa.Name;
                 }
+
+                var parameters = backingMethod.GetParameters().Select(parameter => new UserDataBoundFunctionParameter()
+                {
+                    Name       = parameter.Name,
+                    ActualType = parameter.ParameterType.Name,
+                    Type       = StaticScope.DeriveValidTypes(parameter.ParameterType),
+                    IsOptional = parameter.IsOptional
+                }).ToArray();
                 methods.Add(new UserDataBoundMethod()
                 {
                     Name       = methodName,
                     Type       = Type.Function,
-                    ResultType = backingMethod.ReturnType.GetScriptType()
+                    ResultType = backingMethod.ReturnType.GetScriptType(),
+                    Parameters = parameters
                 });
             }
             foreach (var f in methods)
@@ -123,10 +134,47 @@ namespace Upsilon.BoundTypes
         public string Comment { get; set; }
     }
 
+    public class UserDataBoundFunctionParameter : UserDataBoundProperty
+    {
+        public bool IsOptional { get; set; }
+    }
+
     public class UserDataBoundMethod: UserDataBoundProperty
     {
         public override string ActualType => "Function";
         public Type ResultType { get; set; }
+        public UserDataBoundFunctionParameter[] Parameters { get; set; }
+
+        public (bool IsValid, string Error,
+            BoundExpression WrongParameter) ValidateParameters(ImmutableArray<BoundExpression> callingParameters)
+        {
+            if (callingParameters.Length   < Parameters.Count(x => !x.IsOptional)
+                || callingParameters.Length > Parameters.Length)
+            {
+                return (false,
+                    $"Invalid number of parameters for function '{Name}'. Expected {Parameters.Length}, got {callingParameters.Length}",
+                    null);
+            }
+
+            for (var i = 0; i < callingParameters.Length; i++)
+            {
+                var functionParameter = Parameters[i];
+                var callingParameter  = callingParameters[i];
+                if (callingParameter.Type == Type.Unknown || callingParameter.Type == Type.Nil)
+                    continue;
+
+                if (!functionParameter.Type.HasFlag(callingParameter.Type))
+                {
+                    return (false,
+                        $"Unexpected variable passed to internal function at variable {i + 1}. " +
+                        $"Expected one of the following: {functionParameter.Type.ToString()}, got: '{callingParameter.Type}'",
+                        callingParameter);
+                }
+            }
+
+            return (true, null, null);
+        }
+
     }
 
 }
\ No newline at end of file
diff --git a/Upsilon/StandardLibraries/StaticScope.cs b/Upsilon/StandardLibraries/StaticScope.cs
index 0b1af37..2b0ebdb 100644
--- a/Upsilon/StandardLibraries/StaticScope.cs
+++ b/Upsilon/StandardLibraries/StaticScope.cs
@@ -138,7 +138,7 @@ namespace Upsilon.StandardLibraries
                     new InternalFunctionVariableSymbol.InternalFunctionParameter(t, false)).ToArray());
         }
 
-        private static Type DeriveValidTypes(System.Type type)
+        public static Type DeriveValidTypes(System.Type type)
         {
             var typeCode = System.Type.GetTypeCode(type);
             switch (typeCode)