diff --git a/mod.js b/mod.js
index be6b382..bbd4e62 100644
--- a/mod.js
+++ b/mod.js
@@ -377,7 +377,7 @@ function inputToRGB(color) {
     ) {
       rgb = rgbToRgb(color.r, color.g, color.b);
       ok = true;
-      format = String(color.r).substr(-1) === "%" ? "prgb" : "rgb";
+      format = String(color.r).slice(-1) === "%" ? "prgb" : "rgb";
     } else if (
       isValidCSSUnit(color.h) &&
       isValidCSSUnit(color.s) &&
diff --git a/tinycolor.js b/tinycolor.js
index e52a3d5..74b79d4 100644
--- a/tinycolor.js
+++ b/tinycolor.js
@@ -325,7 +325,7 @@
       if (isValidCSSUnit(color.r) && isValidCSSUnit(color.g) && isValidCSSUnit(color.b)) {
         rgb = rgbToRgb(color.r, color.g, color.b);
         ok = true;
-        format = String(color.r).substr(-1) === "%" ? "prgb" : "rgb";
+        format = String(color.r).slice(-1) === "%" ? "prgb" : "rgb";
       } else if (isValidCSSUnit(color.h) && isValidCSSUnit(color.s) && isValidCSSUnit(color.v)) {
         s = convertToPercentage(color.s);
         v = convertToPercentage(color.v);