diff --git a/beanquery/compiler.py b/beanquery/compiler.py
index d9761cc..390f5f6 100644
--- a/beanquery/compiler.py
+++ b/beanquery/compiler.py
@@ -525,7 +525,7 @@ def _unaryop(self, node: ast.UnaryOp):
         function = types.function_lookup(OPERATORS, type(node), [operand])
         if function is None:
             raise CompilationError(
-                f'operator "{type(node).__name__.lower()}({operand.dtype.__name__})" not supported', node)
+                f'operator "{type(node).__name__.lower()}({types.name(operand.dtype)})" not supported', node)
         function = function(operand)
         # Constants folding.
         if isinstance(operand, EvalConstant):
@@ -593,7 +593,7 @@ def _binaryop(self, node: ast.BinaryOp):
 
         raise CompilationError(
             f'operator "{type(node).__name__.lower()}('
-            f'{left.dtype.__name__}, {right.dtype.__name__})" not supported', node)
+            f'{types.name(left.dtype)}, {types.name(right.dtype)})" not supported', node)
 
     @_compile.register
     def _constant(self, node: ast.Constant):
diff --git a/beanquery/types.py b/beanquery/types.py
index 753252f..baa9dac 100644
--- a/beanquery/types.py
+++ b/beanquery/types.py
@@ -90,4 +90,6 @@ def function_lookup(functions, name, operands):
 
 
 def name(datatype):
+    if datatype is NoneType:
+        return 'NULL'
     return getattr(datatype, 'name', datatype.__name__.lower())