diff --git a/missingno/missingno.py b/missingno/missingno.py
index 7de259d..581e6a4 100644
--- a/missingno/missingno.py
+++ b/missingno/missingno.py
@@ -395,7 +395,11 @@ def heatmap(
     ax0.patch.set_visible(False)
 
     for text in ax0.texts:
-        t = float(text.get_text())
+        if text.get_text() != '--':
+            t = float(text.get_text())
+        else:
+            continue
+        
         if 0.95 <= t < 1:
             text.set_text('<1')
         elif -1 < t <= -0.95: