diff --git a/dowhy/utils/networkx_plotting.py b/dowhy/utils/networkx_plotting.py
index efafa4547..06f97c89d 100644
--- a/dowhy/utils/networkx_plotting.py
+++ b/dowhy/utils/networkx_plotting.py
@@ -32,7 +32,7 @@ def plot_causal_graph_networkx(
         if (source, target) not in causal_strengths:
             causal_strengths[(source, target)] = strength
 
-        if strength is not None:
+        if causal_strengths[(source, target)] is not None:
             max_strength = max(max_strength, abs(causal_strengths[(source, target)]))
 
         if (source, target) not in colors:
diff --git a/dowhy/utils/plotting.py b/dowhy/utils/plotting.py
index 93d37eaf7..2f67001cc 100644
--- a/dowhy/utils/plotting.py
+++ b/dowhy/utils/plotting.py
@@ -176,6 +176,11 @@ def bar_plot(
 
 
 def _calc_arrow_width(strength: float, max_strength: float):
+    if max_strength == 0:
+        return 4.1
+    elif max_strength < 0:
+        raise ValueError("Got a negative strength! The strength needs to be positive.")
+
     return 0.1 + 4.0 * float(abs(strength)) / float(max_strength)
 
 
diff --git a/tests/utils/test_plotting.py b/tests/utils/test_plotting.py
index 880e34b56..ccc6d75d2 100644
--- a/tests/utils/test_plotting.py
+++ b/tests/utils/test_plotting.py
@@ -1,9 +1,11 @@
 import networkx as nx
 import numpy as np
 import pandas as pd
+import pytest
 from _pytest.python_api import approx
 
 from dowhy.utils import plot, plot_adjacency_matrix
+from dowhy.utils.networkx_plotting import plot_causal_graph_networkx
 from dowhy.utils.plotting import _calc_arrow_width, bar_plot
 
 
@@ -48,6 +50,10 @@ def test_calc_arrow_width():
     assert _calc_arrow_width(0.5, max_strength=0.5) == approx(4.1, abs=0.01)
     assert _calc_arrow_width(0.35, max_strength=0.5) == approx(2.9, abs=0.01)
     assert _calc_arrow_width(100, max_strength=101) == approx(4.06, abs=0.01)
+    assert _calc_arrow_width(100, max_strength=0) == 4.1
+
+    with pytest.raises(ValueError):
+        _calc_arrow_width(100, max_strength=-1)
 
 
 def test_given_misspecified_uncertainties_when_bar_plot_then_does_not_raise_error():