diff --git a/pybtc/tools.py b/pybtc/tools.py
index 459b890..0609ab7 100644
--- a/pybtc/tools.py
+++ b/pybtc/tools.py
@@ -104,11 +104,14 @@ def private_to_public_key(private_key, compressed=True, hex=False):
 def is_valid_public_key(key):
     if len(key) < 33:
         return False
-    if key[0] == 0x04 and len(key) != 65:
+    elif key[0] == 0x04 and len(key) != 65:
         return False
     elif key[0] == 0x02 or key[0] == 0x03:
         if len(key) != 33:
             return False
+    elif isinstance(key, str):
+        if not key[:4] in ['xpub', 'tpub']:
+            return False
     return True