diff --git a/kandinsky/include/kandinsky/font.h b/kandinsky/include/kandinsky/font.h
index 3eae2a3b1..d0031c9ba 100644
--- a/kandinsky/include/kandinsky/font.h
+++ b/kandinsky/include/kandinsky/font.h
@@ -40,7 +40,13 @@ private:
     return static_cast<uint8_t>(c) - k_magicCharOffsetValue;
   }
   int signedCharAsIndex(char c) const {
-    return static_cast<int>(c) - k_magicCharOffsetValue;
+    int cInt = c;
+    if (cInt < 0) {
+      /* A char casted as int takes its value between -127 and +128, but we want
+       * a positive value. -127 is thus 129, -126 is 130, etc. */
+      cInt+=128+(cInt-(-127)+1);
+    }
+    return cInt - k_magicCharOffsetValue;
   }
 
   KDSize m_glyphSize;