diff --git a/tabulate/__init__.py b/tabulate/__init__.py
index 3b1a1e1..afbd46a 100644
--- a/tabulate/__init__.py
+++ b/tabulate/__init__.py
@@ -850,7 +850,7 @@ def _isnumber(string):
     >>> _isnumber("inf")
     True
     """
-    if not _isconvertible(float, string):
+    if not _isconvertible(float, str(string)):
         return False
     elif isinstance(string, (str, bytes)) and (
         math.isinf(float(string)) or math.isnan(float(string))