diff --git a/noid.py b/noid.py
index 6277354..38b2bcb 100644
--- a/noid.py
+++ b/noid.py
@@ -182,7 +182,17 @@ class Minter(Persistent):
         # 7. init checkchar to the value of xdig[sum % alphacount]
         # 8. return the concat of id and checkchar is lastchar equal to '+' or checkchar
         # 9. return None
-        pass
+        if id is None:
+            return None
+        lastchar = id[-1]
+        sum = 0
+        for idx, char in enumerate(id[:-1], 1):
+            sum += idx * (self.legalstring.index(char) if self.legalstring.index(char) else 0)
+        checkchar = self.xdig[sum % self.alphacount]
+        if lastchar == "+" or lastchar == checkchar:
+            return id[:-1] + checkchar
+        else:
+            return None
 
 
 if __name__ == "__main__":