diff --git a/fpdf.go b/fpdf.go
index ab06c4a..e314199 100644
--- a/fpdf.go
+++ b/fpdf.go
@@ -126,7 +126,7 @@ func fpdfNew(orientationStr, unitStr, sizeStr, fontDirStr string, size SizeType)
 	case "pt", "point":
 		f.k = 1.0
 	case "mm":
-		a = 72.0 / 25.4
+		f.k = 72.0 / 25.4
 	case "cm":
 		f.k = 72.0 / 2.54
 	case "in", "inch":