diff --git a/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableArray_1.Builder.cs b/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableArray_1.Builder.cs
index 4de80f12537927..7efb68bfc658bb 100644
--- a/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableArray_1.Builder.cs
+++ b/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableArray_1.Builder.cs
@@ -793,12 +793,15 @@ public int IndexOf(T item, int startIndex, int count)
             /// <returns>The 0-based index into the array where the item was found; or -1 if it could not be found.</returns>
             public int IndexOf(T item, int startIndex, int count, IEqualityComparer<T>? equalityComparer)
             {
-                if (count == 0 && startIndex == 0)
+                if (count == 0)
                 {
-                    return -1;
+                    if (startIndex == 0 || startIndex == this.Count)
+                    {
+                        return -1;
+                    }
                 }
 
-                Requires.Range(startIndex >= 0 && startIndex < this.Count, nameof(startIndex));
+                Requires.Range(startIndex >= 0 && startIndex <= this.Count, nameof(startIndex));
                 Requires.Range(count >= 0 && startIndex + count <= this.Count, nameof(count));
 
                 equalityComparer ??= EqualityComparer<T>.Default;
diff --git a/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableArray_1.cs b/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableArray_1.cs
index ec4d94dc62184e..5dc90a900bc6c0 100644
--- a/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableArray_1.cs
+++ b/src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableArray_1.cs
@@ -168,12 +168,15 @@ public int IndexOf(T item, int startIndex, int count, IEqualityComparer<T>? equa
             ImmutableArray<T> self = this;
             self.ThrowNullRefIfNotInitialized();
 
-            if (count == 0 && startIndex == 0)
+            if (count == 0)
             {
-                return -1;
+                if (startIndex == 0 || startIndex == self.Length)
+                {
+                    return -1;
+                }
             }
 
-            Requires.Range(startIndex >= 0 && startIndex < self.Length, nameof(startIndex));
+            Requires.Range(startIndex >= 0 && startIndex <= self.Length, nameof(startIndex));
             Requires.Range(count >= 0 && startIndex + count <= self.Length, nameof(count));
 
             equalityComparer ??= EqualityComparer<T>.Default;
@@ -253,12 +256,15 @@ public int LastIndexOf(T item, int startIndex, int count, IEqualityComparer<T>?
             ImmutableArray<T> self = this;
             self.ThrowNullRefIfNotInitialized();
 
-            if (startIndex == 0 && count == 0)
+            if (count == 0)
             {
-                return -1;
+                if (startIndex == 0 || startIndex == self.Length)
+                {
+                    return -1;
+                }
             }
 
-            Requires.Range(startIndex >= 0 && startIndex < self.Length, nameof(startIndex));
+            Requires.Range(startIndex >= 0 && startIndex <= self.Length, nameof(startIndex));
             Requires.Range(count >= 0 && startIndex - count + 1 >= 0, nameof(count));
 
             equalityComparer ??= EqualityComparer<T>.Default;
diff --git a/src/libraries/System.Collections.Immutable/tests/ImmutableArrayTest.cs b/src/libraries/System.Collections.Immutable/tests/ImmutableArrayTest.cs
index 80a3ec1a37e6f0..6d8f2b83510033 100644
--- a/src/libraries/System.Collections.Immutable/tests/ImmutableArrayTest.cs
+++ b/src/libraries/System.Collections.Immutable/tests/ImmutableArrayTest.cs
@@ -2855,5 +2855,28 @@ public StructWithReferenceTypeField(string foo)
                 this.foo = foo;
             }
         }
+        
+        [Theory]
+        [InlineData(0, 0)]   // startIndex 0, count 0
+        [InlineData(4, 0)]   // startIndex == Length, count 0
+        [InlineData(2, 0)]   // intermediate startIndex, count 0
+        public void IndexOf_CountZero_ReturnsMinusOne(int startIndex, int count)
+        {
+            var array = ImmutableArray.Create(1, 2, 3, 4);
+            Assert.Equal(-1, array.IndexOf(2, startIndex, count));
+        }
+
+        [Fact]
+        public void IndexOf_EmptyArray_CountZero_ReturnsMinusOne()
+        {
+            Assert.Equal(-1, ImmutableArray<int>.Empty.IndexOf(1, 0, 0));
+        }
+
+        [Fact]
+        public void IndexOf_Builder_CountZero_ReturnsMinusOne()
+        {
+            var builder = ImmutableArray.Create(1, 2, 3, 4).ToBuilder();
+            Assert.Equal(-1, builder.IndexOf(2, builder.Count, 0));
+        }
     }
 }