diff --git a/morphisms/digit.morphism-base b/morphisms/digit.morphism-base
index 97d3bd9..8cc1597 100644
--- a/morphisms/digit.morphism-base
+++ b/morphisms/digit.morphism-base
@@ -12,8 +12,10 @@ morph_digit_as_char_to_uint8 (Radix:ℤ)
         *dst = 0xa + *src - 'a';
     else if( *src >= 'A' && *src <= 'F')
         *dst = 0xa + *src - 'A';
-    else
+    else {
+        fprintf(stderr, "invalid digit 0x%x\n", *src);
         return -1;
+    }
 
     if( *dst < Radix ) {
         return 0;
@@ -33,8 +35,10 @@ morph_digit_as_char_to_uint64 (Radix:ℤ)
         *dst = 0xa + *src - 'a';
     else if( *src >= 'A' && *src <= 'F')
         *dst = 0xa + *src - 'A';
-    else
+    else {
+        fprintf(stderr, "invalid digit 0x%x\n", *src);
         return -1;
+    }
 
     if( *dst < Radix ) {
         return 0;