diff --git a/sourcetracker/_plot.py b/sourcetracker/_plot.py
index 9dcfc97..be4c081 100644
--- a/sourcetracker/_plot.py
+++ b/sourcetracker/_plot.py
@@ -241,7 +241,7 @@ def ST_Stacked_bar(self, keep_unknowns=True, x_lab="Sink",
         prop = prop.reset_index()
         if len(coloring) != (prop.shape[1]-1):
             coloring = []
-        if coloring = []:
+        if coloring == []:
             coloring = None
         prop.plot(kind='bar', x=prop.columns[0], stacked=True,
                      figsize=((prop.shape[1] * 3 / 4)+4,