diff --git a/iyzipay/pki_builder.py b/iyzipay/pki_builder.py
index e49b635..93bfa90 100644
--- a/iyzipay/pki_builder.py
+++ b/iyzipay/pki_builder.py
@@ -8,7 +8,7 @@ def append(self, key, value=None):
         return self
 
     def append_price(self, key, value=None):
-        if value is not None and value is not "":
+        if value is not None and value != "":
             self.append_key_value(key, str(round(float(value), 2)))
         return self
 
@@ -22,7 +22,7 @@ def append_array(self, key, array=None):
         return self
 
     def append_key_value(self, key, value=None):
-        if value is not None and value is not "":
+        if value is not None and value != "":
             self.request_string = self.request_string + key + "=" + str(value) + ","
 
     def remove_trailing_comma(self):