diff --git a/loopy/transform/iname.py b/loopy/transform/iname.py
index 51de1119112b9ce4e311a743d2cd7405398179ed..fe322b9d3f20838ef02fcfd0c59a1a49a8d0dcec 100644
--- a/loopy/transform/iname.py
+++ b/loopy/transform/iname.py
@@ -1287,6 +1287,9 @@ def _split_reduction(kernel, inames, direction, within=None):
         inames = inames.split(",")
     inames = set(inames)
 
+    if not (inames <= kernel.all_inames()):
+        raise LoopyError("Unknown inames: {}.".format(inames-kernel.all_inames()))
+
     from loopy.match import parse_stack_match
     within = parse_stack_match(within)