diff -r d4a4ba52405d -r 6b7d72a17973 ExternalLibraryFunction.st
--- a/ExternalLibraryFunction.st	Thu Apr 11 07:30:32 2019 +0200
+++ b/ExternalLibraryFunction.st	Thu Apr 11 08:54:15 2019 +0200
@@ -3019,7 +3019,8 @@
 	    ^ self primitiveFailed:'FFI support missing in this build'.
 	].
 	(failureCode == #InvalidArgument) ifTrue:[
-	    ^ self primitiveFailed:('cannot convert argument: #%1' bindWith:failureArgNr).
+	    ^ self primitiveFailed:('cannot convert argument: #%1 (to %2)'
+					bindWith:failureArgNr with:(argTypeSymbols at:failureArgNr)).
 	].
 	(failureCode == #UnknownReturnType) ifTrue:[
 	    ^ self primitiveFailed:('unknown return type: #%1' bindWith:returnTypeSymbol).