diff --git a/Lib/idlelib/idle b/Lib/idlelib/idle
index 3b08cae2347..00a1fe3a00d 100755
--- a/Lib/idlelib/idle
+++ b/Lib/idlelib/idle
@@ -14,7 +14,10 @@ except ImportError:
         if idledir != os.getcwd():
             # We're not in the IDLE directory, help the subprocess find run.py
             pypath = os.environ.get('PYTHONPATH', '')
-            os.environ['PYTHONPATH'] = pypath + ':' + idledir
+            if pypath:
+                os.environ['PYTHONPATH'] = pypath + ':' + idledir
+            else:
+                os.environ['PYTHONPATH'] = idledir
         PyShell.main()
 else:
     idlelib.PyShell.main()
diff --git a/Lib/idlelib/idle.py b/Lib/idlelib/idle.py
index 3b08cae2347..00a1fe3a00d 100644
--- a/Lib/idlelib/idle.py
+++ b/Lib/idlelib/idle.py
@@ -14,7 +14,10 @@
         if idledir != os.getcwd():
             # We're not in the IDLE directory, help the subprocess find run.py
             pypath = os.environ.get('PYTHONPATH', '')
-            os.environ['PYTHONPATH'] = pypath + ':' + idledir
+            if pypath:
+                os.environ['PYTHONPATH'] = pypath + ':' + idledir
+            else:
+                os.environ['PYTHONPATH'] = idledir
         PyShell.main()
 else:
     idlelib.PyShell.main()
diff --git a/Lib/idlelib/idle.pyw b/Lib/idlelib/idle.pyw
index 767db865a55..537dd5a9a75 100644
--- a/Lib/idlelib/idle.pyw
+++ b/Lib/idlelib/idle.pyw
@@ -12,7 +12,10 @@ except ImportError:
         if idledir != os.getcwd():
             # We're not in the IDLE directory, help the subprocess find run.py
             pypath = os.environ.get('PYTHONPATH', '')
-            os.environ['PYTHONPATH'] = pypath + ':' + idledir
+            if pypath:
+                os.environ['PYTHONPATH'] = pypath + ':' + idledir
+            else:
+                os.environ['PYTHONPATH'] = idledir
         PyShell.main()
 else:
     idlelib.PyShell.main()