diff --git a/src/HTMLState.h b/src/HTMLState.h
index b705068..060bfdd 100644
--- a/src/HTMLState.h
+++ b/src/HTMLState.h
@@ -34,8 +34,10 @@ struct HTMLTextState
     
     // the offset cause by a single ' ' char
     double single_space_offset(void) const {
-        assert(font_info->em_size != 0);
-        return word_space + letter_space + font_info->space_width / font_info->em_size * font_size;
+        double offset = word_space + letter_space;
+        if(font_info->em_size != 0)
+            offset += font_info->space_width / font_info->em_size * font_size;
+        return offset;
     }
     // calculate em_size of this state
     double em_size(void) const {