diff --git a/WDL/Error.py b/WDL/Error.py
index d9b730a1..5f9e48b4 100644
--- a/WDL/Error.py
+++ b/WDL/Error.py
@@ -185,10 +185,12 @@ def __init__(self, node: SourceNode, message: str) -> None:
 
 
 class UnknownIdentifier(ValidationError):
-    def __init__(self, node: SourceNode) -> None:
+    def __init__(self, node: SourceNode, message: Optional[str] = None) -> None:
         # avoiding circular dep:
         # assert isinstance(node, WDL.Expr.Ident)
-        super().__init__(node, "Unknown identifier " + str(node))
+        if not message:
+            message = "Unknown identifier " + str(node)
+        super().__init__(node, message)
 
 
 class NoSuchInput(ValidationError):
diff --git a/WDL/Expr.py b/WDL/Expr.py
index ff2f3784..ad4780bf 100644
--- a/WDL/Expr.py
+++ b/WDL/Expr.py
@@ -956,7 +956,11 @@ def _infer_type(self, type_env: Env.Bindings[Type.Base]) -> Type.Base:
             # attempt to resolve "expr.member" and if that works, transform
             # expr to Ident("expr.member")
             if self.expr._ident + "." + self.member not in type_env:
-                raise Error.UnknownIdentifier(self) from None
+                message = None
+                if type_env.has_namespace(self.expr._ident):
+                    # specific error message when namespace exists but member doesn't
+                    message = f"No {self.member} in namespace {self.expr._ident}"
+                raise Error.UnknownIdentifier(self, message=message) from None
             self.expr = Ident(self.pos, self._ident)
             self.expr.infer_type(type_env, self._stdlib, self._check_quant)
             self.member = None
diff --git a/tests/test_1doc.py b/tests/test_1doc.py
index 259bfe19..20c24ac7 100644
--- a/tests/test_1doc.py
+++ b/tests/test_1doc.py
@@ -811,7 +811,7 @@ def test_errors(self):
         }
         """
         doc = WDL.parse_document(doc)
-        with self.assertRaises(WDL.Error.UnknownIdentifier):
+        with self.assertRaisesRegex(WDL.Error.UnknownIdentifier, " in namespace "):
             doc.typecheck()
 
         doc = r"""