diff --git a/src/nimly/lexgen.nim b/src/nimly/lexgen.nim
index 1141976..bc5e0e9 100644
--- a/src/nimly/lexgen.nim
+++ b/src/nimly/lexgen.nim
@@ -816,7 +816,7 @@ proc convertToSeqRePart(re: string): seq[RePart] =
         of 'd', 'D', 's', 'S', 'w', 'W':
           result.add(Tree(classTable[c].convertToTree))
         else:
-          doassert false, "Invalid escaping"
+          doassert false, "Invalid escaping \"\\" & c & "\" is in niml match pattern."
         flag.excl(readingEscape)
       else:
         case c
diff --git a/tests/ex.nim b/tests/ex.nim
new file mode 100644
index 0000000..b2c0325
--- /dev/null
+++ b/tests/ex.nim
@@ -0,0 +1,9 @@
+import nimly
+import patty
+
+variant Token:
+  FOO
+
+niml a[Token]:
+  r"\:":
+    return FOO()