diff --git a/src/libraries/System.Private.CoreLib/src/System/AppContext.cs b/src/libraries/System.Private.CoreLib/src/System/AppContext.cs
index b7756bb3f6669e..d0ab3305bfd4bc 100644
--- a/src/libraries/System.Private.CoreLib/src/System/AppContext.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/AppContext.cs
@@ -141,24 +141,24 @@ public static void SetSwitch(string switchName, bool isEnabled)
             }
         }
 
-#if !NATIVEAOT
-        internal static unsafe void Setup(char** pNames, char** pValues, int count)
+#if MONO
+        internal static unsafe void Setup(char** pNames, uint* pNameLengths, char** pValues, uint* pValueLengths, int count)
         {
             Debug.Assert(s_dataStore == null, "s_dataStore is not expected to be inited before Setup is called");
             s_dataStore = new Dictionary<string, object?>(count);
             for (int i = 0; i < count; i++)
             {
-                s_dataStore.Add(new string(pNames[i]), new string(pValues[i]));
+                s_dataStore.Add(new string(pNames[i], 0, (int)pNameLengths[i]), new string(pValues[i], 0, (int)pValueLengths[i]));
             }
         }
-
-        internal static unsafe void Setup(char** pNames, uint* pNameLengths, char** pValues, uint* pValueLengths, int count)
+#elif !NATIVEAOT
+        internal static unsafe void Setup(char** pNames, char** pValues, int count)
         {
             Debug.Assert(s_dataStore == null, "s_dataStore is not expected to be inited before Setup is called");
             s_dataStore = new Dictionary<string, object?>(count);
             for (int i = 0; i < count; i++)
             {
-                s_dataStore.Add(new string(pNames[i], 0, (int)pNameLengths[i]), new string(pValues[i], 0, (int)pValueLengths[i]));
+                s_dataStore.Add(new string(pNames[i]), new string(pValues[i]));
             }
         }
 #endif