diff --git a/packages/mermaid/src/themes/theme-base.js b/packages/mermaid/src/themes/theme-base.js
index fff03ff14..01f8a9c0b 100644
--- a/packages/mermaid/src/themes/theme-base.js
+++ b/packages/mermaid/src/themes/theme-base.js
@@ -212,7 +212,7 @@ class Theme {
     this.pieLegendTextColor = this.pieLegendTextColor || this.taskTextDarkColor;
     this.pieStrokeColor = this.pieStrokeColor || 'black';
     this.pieStrokeWidth = this.pieStrokeWidth || '2px';
-    this.pieOuterStrokeWidth = this.pieOuterStrokeWidth ?? '2px';
+    this.pieOuterStrokeWidth = this.pieOuterStrokeWidth || '2px';
     this.pieOuterStrokeColor = this.pieOuterStrokeColor || 'black';
     this.pieOpacity = this.pieOpacity || '0.7';
 
diff --git a/packages/mermaid/src/themes/theme-dark.js b/packages/mermaid/src/themes/theme-dark.js
index b77f0e569..9585a2e27 100644
--- a/packages/mermaid/src/themes/theme-dark.js
+++ b/packages/mermaid/src/themes/theme-dark.js
@@ -222,7 +222,7 @@ class Theme {
     this.pieLegendTextColor = this.pieLegendTextColor || this.taskTextDarkColor;
     this.pieStrokeColor = this.pieStrokeColor || 'black';
     this.pieStrokeWidth = this.pieStrokeWidth || '2px';
-    this.pieOuterStrokeWidth = this.pieOuterStrokeWidth ?? '2px';
+    this.pieOuterStrokeWidth = this.pieOuterStrokeWidth || '2px';
     this.pieOuterStrokeColor = this.pieOuterStrokeColor || 'black';
     this.pieOpacity = this.pieOpacity || '0.7';
 
diff --git a/packages/mermaid/src/themes/theme-default.js b/packages/mermaid/src/themes/theme-default.js
index 74b972edc..c91029de3 100644
--- a/packages/mermaid/src/themes/theme-default.js
+++ b/packages/mermaid/src/themes/theme-default.js
@@ -244,7 +244,7 @@ class Theme {
     this.pieLegendTextColor = this.pieLegendTextColor || this.taskTextDarkColor;
     this.pieStrokeColor = this.pieStrokeColor || 'black';
     this.pieStrokeWidth = this.pieStrokeWidth || '2px';
-    this.pieOuterStrokeWidth = this.pieOuterStrokeWidth ?? '2px';
+    this.pieOuterStrokeWidth = this.pieOuterStrokeWidth || '2px';
     this.pieOuterStrokeColor = this.pieOuterStrokeColor || 'black';
     this.pieOpacity = this.pieOpacity || '0.7';
 
diff --git a/packages/mermaid/src/themes/theme-forest.js b/packages/mermaid/src/themes/theme-forest.js
index d0e1565de..96d6c35c1 100644
--- a/packages/mermaid/src/themes/theme-forest.js
+++ b/packages/mermaid/src/themes/theme-forest.js
@@ -213,7 +213,7 @@ class Theme {
     this.pieLegendTextColor = this.pieLegendTextColor || this.taskTextDarkColor;
     this.pieStrokeColor = this.pieStrokeColor || 'black';
     this.pieStrokeWidth = this.pieStrokeWidth || '2px';
-    this.pieOuterStrokeWidth = this.pieOuterStrokeWidth ?? '2px';
+    this.pieOuterStrokeWidth = this.pieOuterStrokeWidth || '2px';
     this.pieOuterStrokeColor = this.pieOuterStrokeColor || 'black';
     this.pieOpacity = this.pieOpacity || '0.7';
 
diff --git a/packages/mermaid/src/themes/theme-neutral.js b/packages/mermaid/src/themes/theme-neutral.js
index 34bf5e0da..8bb5ff693 100644
--- a/packages/mermaid/src/themes/theme-neutral.js
+++ b/packages/mermaid/src/themes/theme-neutral.js
@@ -243,7 +243,7 @@ class Theme {
     this.pieLegendTextColor = this.pieLegendTextColor || this.taskTextDarkColor;
     this.pieStrokeColor = this.pieStrokeColor || 'black';
     this.pieStrokeWidth = this.pieStrokeWidth || '2px';
-    this.pieOuterStrokeWidth = this.pieOuterStrokeWidth ?? '2px';
+    this.pieOuterStrokeWidth = this.pieOuterStrokeWidth || '2px';
     this.pieOuterStrokeColor = this.pieOuterStrokeColor || 'black';
     this.pieOpacity = this.pieOpacity || '0.7';