diff --git a/src/sprintf.js b/src/sprintf.js
index 65d6324..03017dd 100644
--- a/src/sprintf.js
+++ b/src/sprintf.js
@@ -63,7 +63,7 @@
                 }
 
                 if (re.number.test(ph.type)) {
-                    is_positive = arg >= 0
+                    is_positive = arg >= 0 || isNaN(arg)
                 }
 
                 switch (ph.type) {
diff --git a/test/test.js b/test/test.js
index 9588da6..df236ab 100644
--- a/test/test.js
+++ b/test/test.js
@@ -41,6 +41,7 @@ describe('sprintfjs', function() {
         assert.equal('f', sprintf('%.1t', false))
         assert.equal('false', sprintf('%t', ''))
         assert.equal('false', sprintf('%t', 0))
+        assert.equal('NaN', sprintf('%f', NaN))
 
         assert.equal('undefined', sprintf('%T', undefined))
         assert.equal('null', sprintf('%T', null))