add unitless if zero

index.js

@@ -60,7 +60,8 @@ function createPxReplace(viewportSize, minPixelValue, unitPrecision, viewportUni
     if (!$1) return m;
     var pixels = parseFloat($1);
     if (pixels <= minPixelValue) return m;
-    return toFixed((pixels / viewportSize * 100), unitPrecision) + viewportUnit;
+    var parsedVal = toFixed((pixels / viewportSize * 100), unitPrecision);
+    return parsedVal === 0 ? '0' : parsedVal + viewportUnit;
   };
 }
 

spec/px-to-viewport.spec.js

@@ -36,6 +36,13 @@ describe('px-to-viewport', function() {
 
     expect(processed).toBe(expected);
   });
+
+  it('should remain unitless if 0', function () {
+    var expected = '.rule { font-size: 0px; font-size: 0; }';
+    var processed = postcss(pxToViewport()).process(expected).css;
+
+    expect(processed).toBe(expected);
+  });
 });
 
 describe('value parsing', function() {