--- a/frames/iframemul.m
+++ b/frames/iframemul.m
@@ -50,7 +50,19 @@
-% TODO: Check that the symbol length match the input signal length
+% Check for compatibility
+if L1~=L2
+    error(['%s: The symbol and signal lengths are incompatible.'],upper(mfilename));
+% This is not *strictly* necessary, but we cannot check that the symbol
+% is complex-valued in just the right way.
+if Fa.realinput && ~isreal(s)
+    error(['%s: For real-valued-input-only frames, the symbol must also ' ...
+           'be real.'],upper(mfilename));
 % The frame multiplier is not positive definite, so we cannot solve it
 % directly using pcg.