diff --git a/src/core/CharacterWidthDeterminer.h b/src/core/CharacterWidthDeterminer.h
index 4f9b83b..6a90ce0 100644
--- a/src/core/CharacterWidthDeterminer.h
+++ b/src/core/CharacterWidthDeterminer.h
@@ -6,7 +6,7 @@
 class CharacterWidthDeterminer
 {
 public:
-    virtual int operator()(uint32_t character);
+    virtual int operator()(uint32_t character) = 0;
 };
 
 #endif