diff --git a/NanoVNASaver/Chart.py b/NanoVNASaver/Chart.py
index 2976217..c18b901 100644
--- a/NanoVNASaver/Chart.py
+++ b/NanoVNASaver/Chart.py
@@ -3839,7 +3839,7 @@ class GroupDelayChart(FrequencyChart):
         for d in self.reference:
             rawReference.append(d.phase)
 
-        if len(self.data) > 0:
+        if len(self.data) > 1:
             unwrappedData = np.degrees(np.unwrap(rawData))
             self.groupDelay = []
             for i in range(len(self.data)):
@@ -3859,7 +3859,7 @@ class GroupDelayChart(FrequencyChart):
                     delay /= 2
                 self.groupDelay.append(delay)
 
-        if len(self.reference) > 0:
+        if len(self.reference) > 1:
             unwrappedReference = np.degrees(np.unwrap(rawReference))
             self.groupDelayReference = []
             for i in range(len(self.reference)):
diff --git a/test/data/bug_159.s1p b/test/data/bug_159.s1p
new file mode 100644
index 0000000..7c0cf51
--- /dev/null
+++ b/test/data/bug_159.s1p
@@ -0,0 +1,2 @@
+# MHz S RI R 50
+28.000000 0.0 0.0