diff --git a/src/fontcache.cpp b/src/fontcache.cpp
index 2c1f7aafa2..b51cecaf6c 100644
--- a/src/fontcache.cpp
+++ b/src/fontcache.cpp
@@ -76,7 +76,7 @@ public:
 	virtual const Sprite *GetGlyph(GlyphID key);
 	virtual uint GetGlyphWidth(GlyphID key);
 	virtual bool GetDrawGlyphShadow();
-	virtual GlyphID MapCharToGlyph(WChar key) { return SPRITE_GLYPH | key; }
+	virtual GlyphID MapCharToGlyph(WChar key) { assert(IsPrintable(key)); return SPRITE_GLYPH | key; }
 	virtual const void *GetFontTable(uint32 tag, size_t &length) { length = 0; return NULL; }
 };