diff --git a/koala/Range.py b/koala/Range.py
index <HASH>..<HASH> 100644
--- a/koala/Range.py
+++ b/koala/Range.py
@@ -539,7 +539,12 @@ class RangeCore(dict):
     @staticmethod
     def add(a, b):
         try:
-            return check_value(a) + check_value(b)
+            a = check_value(a)
+            b = check_value(b)
+            if isinstance(a, str) or isinstance(b, str):
+                a = str(a)
+                b = str(b)
+            return a + b
         except Exception as e:
             return ExcelError('#N/A', e)
Add strings

If one of the two values would be a string like in the formula "'<='&<I>", this would crash since Python cannot add int + str. If this is the case, parse them both as strings.