diff --git a/laserbeamsize/analysis.py b/laserbeamsize/analysis.py
index 106fdea..508ec0b 100644
--- a/laserbeamsize/analysis.py
+++ b/laserbeamsize/analysis.py
@@ -202,7 +202,7 @@ def beam_size(image,
 
     for _iteration in range(1, max_iter):
 
-        phi_ = phi or phi_
+        phi_ = phi if phi is not None else phi_
 
         # save current beam properties for later comparison
         xc2, yc2, dx2, dy2 = xc, yc, dx, dy
@@ -220,8 +220,7 @@ def beam_size(image,
         if abs(xc - xc2) < 1 and abs(yc - yc2) < 1 and abs(dx - dx2) < 1 and abs(dy - dy2) < 1:
             break
 
-    phi_ = phi or phi_
-
+    phi_ = phi if phi is not None else phi_
     return xc, yc, dx, dy, phi_