diff --git a/src/ImageSharp/Advanced/ParallelUtils/ParallelHelper.cs b/src/ImageSharp/Advanced/ParallelUtils/ParallelHelper.cs
index ecefadb08d..0414f3ae3f 100644
--- a/src/ImageSharp/Advanced/ParallelUtils/ParallelHelper.cs
+++ b/src/ImageSharp/Advanced/ParallelUtils/ParallelHelper.cs
@@ -62,14 +62,23 @@ namespace SixLabors.ImageSharp.Advanced.ParallelUtils
 
             var parallelOptions = new ParallelOptions { MaxDegreeOfParallelism = numOfSteps };
 
+            int top = rectangle.Top;
+            int bottom = rectangle.Bottom;
+
             Parallel.For(
                 0,
                 numOfSteps,
                 parallelOptions,
                 i =>
                     {
-                        int yMin = rectangle.Top + (i * verticalStep);
-                        int yMax = Math.Min(yMin + verticalStep, rectangle.Bottom);
+                        int yMin = top + (i * verticalStep);
+
+                        if (yMin >= bottom)
+                        {
+                            return;
+                        }
+
+                        int yMax = Math.Min(yMin + verticalStep, bottom);
 
                         var rows = new RowInterval(yMin, yMax);
                         body(rows);
@@ -110,13 +119,22 @@ namespace SixLabors.ImageSharp.Advanced.ParallelUtils
 
             var parallelOptions = new ParallelOptions { MaxDegreeOfParallelism = numOfSteps };
 
+            int top = rectangle.Top;
+            int bottom = rectangle.Bottom;
+
             Parallel.For(
                 0,
                 numOfSteps,
                 parallelOptions,
                 i =>
                     {
-                        int yMin = rectangle.Top + (i * verticalStep);
+                        int yMin = top + (i * verticalStep);
+
+                        if (yMin >= bottom)
+                        {
+                            return;
+                        }
+
                         int yMax = Math.Min(yMin + verticalStep, rectangle.Bottom);
 
                         var rows = new RowInterval(yMin, yMax);
diff --git a/tests/ImageSharp.Tests/Helpers/ParallelHelperTests.cs b/tests/ImageSharp.Tests/Helpers/ParallelHelperTests.cs
index 5914aba400..69942eef98 100644
--- a/tests/ImageSharp.Tests/Helpers/ParallelHelperTests.cs
+++ b/tests/ImageSharp.Tests/Helpers/ParallelHelperTests.cs
@@ -28,17 +28,18 @@ namespace SixLabors.ImageSharp.Tests.Helpers
         /// <summary>
         /// maxDegreeOfParallelism, minY, maxY, expectedStepLength, expectedLastStepLength
         /// </summary>
-        public static TheoryData<int, int, int, int, int> IterateRows_OverMinimumPixelsLimit_Data =
-            new TheoryData<int, int, int, int, int>
+        public static TheoryData<int, int, int, int, int, int> IterateRows_OverMinimumPixelsLimit_Data =
+            new TheoryData<int, int, int, int, int, int>
                 {
-                    { 1, 0, 100, -1, 100 },
-                    { 2, 0, 9, 5, 4 },
-                    { 4, 0, 19, 5, 4 },
-                    { 2, 10, 19, 5, 4 },
-                    { 4, 0, 200, 50, 50 },
-                    { 4, 123, 323, 50, 50 },
-                    { 4, 0, 1201, 301, 298 },
-                    { 8, 10, 236, 29, 23 }
+                    { 1, 0, 100, -1, 100, 1 },
+                    { 2, 0, 9, 5, 4, 2 },
+                    { 4, 0, 19, 5, 4, 4 },
+                    { 2, 10, 19, 5, 4, 2 },
+                    { 4, 0, 200, 50, 50, 4 },
+                    { 4, 123, 323, 50, 50, 4 },
+                    { 4, 0, 1201, 301, 298, 4 },
+                    { 8, 10, 236, 29, 23, 8 },
+                    { 16, 0, 209, 14, 13, 15 }
                 };
 
         [Theory]
@@ -48,7 +49,8 @@ namespace SixLabors.ImageSharp.Tests.Helpers
             int minY,
             int maxY,
             int expectedStepLength,
-            int expectedLastStepLength)
+            int expectedLastStepLength,
+            int expectedNumberOfSteps)
         {
             var parallelSettings = new ParallelExecutionSettings(
                 maxDegreeOfParallelism,
@@ -74,7 +76,7 @@ namespace SixLabors.ImageSharp.Tests.Helpers
                         Assert.Equal(expected, step);
                     });
 
-            Assert.Equal(maxDegreeOfParallelism, actualNumberOfSteps);
+            Assert.Equal(expectedNumberOfSteps, actualNumberOfSteps);
         }
 
         [Theory]
@@ -84,7 +86,8 @@ namespace SixLabors.ImageSharp.Tests.Helpers
             int minY,
             int maxY,
             int expectedStepLength,
-            int expectedLastStepLength)
+            int expectedLastStepLength,
+            int expectedNumberOfSteps)
         {
             var parallelSettings = new ParallelExecutionSettings(
                 maxDegreeOfParallelism,
@@ -117,7 +120,8 @@ namespace SixLabors.ImageSharp.Tests.Helpers
             int minY,
             int maxY,
             int expectedStepLength,
-            int expectedLastStepLength)
+            int expectedLastStepLength,
+            int expectedNumberOfSteps)
         {
             var parallelSettings = new ParallelExecutionSettings(
                 maxDegreeOfParallelism,
@@ -146,7 +150,7 @@ namespace SixLabors.ImageSharp.Tests.Helpers
                         Assert.Equal(expected, step);
                     });
 
-            Assert.Equal(maxDegreeOfParallelism, actualNumberOfSteps);
+            Assert.Equal(expectedNumberOfSteps, actualNumberOfSteps);
 
             int numberOfDifferentBuffers = bufferHashes.Distinct().Count();
             Assert.Equal(actualNumberOfSteps, numberOfDifferentBuffers);
@@ -159,7 +163,8 @@ namespace SixLabors.ImageSharp.Tests.Helpers
             int minY,
             int maxY,
             int expectedStepLength,
-            int expectedLastStepLength)
+            int expectedLastStepLength,
+            int expectedNumberOfSteps)
         {
             var parallelSettings = new ParallelExecutionSettings(
                 maxDegreeOfParallelism,