diff --git a/compiler/semexprs.nim b/compiler/semexprs.nim
index 2c1d40ff2523f..de2259eb4f786 100644
--- a/compiler/semexprs.nim
+++ b/compiler/semexprs.nim
@@ -361,6 +361,8 @@ proc semCast(c: PContext, n: PNode): PNode =
   let castedExpr = semExprWithType(c, n[1])
   if castedExpr.kind == nkClosedSymChoice:
     errorUseQualifier(c, n[1].info, castedExpr)
+  if targetType == nil:
+    localError(c.config, n.info, "Invalid usage of cast, cast requires a type to convert to, e.g., cast[int](0d).")
   if tfHasMeta in targetType.flags:
     localError(c.config, n[0].info, "cannot cast to a non concrete type: '$1'" % $targetType)
   if not isCastable(c, targetType, castedExpr.typ, n.info):
diff --git a/tests/types/t21027.nim b/tests/types/t21027.nim
new file mode 100644
index 0000000000000..3a992177a43a1
--- /dev/null
+++ b/tests/types/t21027.nim
@@ -0,0 +1,5 @@
+discard """
+  errormsg: "Invalid usage of cast, cast requires a type to convert to, e.g., cast[int](0d)."
+"""
+# bug #21027
+let x: uint64 = cast(5)