diff --git a/src/Hl7.Fhir.Base/FhirPath/Functions/StringOperators.cs b/src/Hl7.Fhir.Base/FhirPath/Functions/StringOperators.cs
index 35424baefe..6f21c0898b 100644
--- a/src/Hl7.Fhir.Base/FhirPath/Functions/StringOperators.cs
+++ b/src/Hl7.Fhir.Base/FhirPath/Functions/StringOperators.cs
@@ -20,7 +20,12 @@ internal static class StringOperators
     {
         public static string FpSubstring(this string me, long start, long? length)
         {
-            var l = length ?? me.Length;
+            var l = length switch
+            {
+                < 0 => 0,
+                null => me.Length,
+                _ => length.Value
+            };
 
             if (start < 0 || start >= me.Length) return null;
             l = Math.Min(l, me.Length - start);
@@ -30,7 +35,7 @@ public static string FpSubstring(this string me, long start, long? length)
 
         public static ITypedElement FpIndexOf(this string me, string fragment)
         {
-            return ElementNode.ForPrimitive(me.IndexOf(fragment));
+            return ElementNode.ForPrimitive(me.IndexOf(fragment, StringComparison.Ordinal));
         }
 
         public static IEnumerable<ITypedElement> ToChars(this string me) =>
diff --git a/src/Hl7.FhirPath.Tests/Functions/StringFunctionsTests.cs b/src/Hl7.FhirPath.Tests/Functions/StringFunctionsTests.cs
index effacb9451..3c4dbc38de 100644
--- a/src/Hl7.FhirPath.Tests/Functions/StringFunctionsTests.cs
+++ b/src/Hl7.FhirPath.Tests/Functions/StringFunctionsTests.cs
@@ -3,6 +3,7 @@
 using Hl7.FhirPath.Functions;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using System;
+using System.Collections.Generic;
 using System.Linq;
 
 namespace HL7.FhirPath.Tests.Functions
@@ -25,6 +26,22 @@ public void StringSplit()
             Assert.AreEqual(5, StringOperators.FpSplit("Peter,James,Jim,Peter,James", ",").Count());
         }
 
+        private static IEnumerable<object[]> substringTests()
+        {
+            yield return ["Ewout", 0, (long?)3, "Ewo"];
+            yield return ["Ewout", 2, (long?)3, "out"];
+            yield return ["Ewout", 0, null, "Ewout"];
+            yield return ["Ewout", 0, (long?)0, ""];
+            yield return ["Ewout", 0, (long?)-1, ""];
+        }
+        
+        [DataTestMethod]
+        [DynamicData(nameof(substringTests), DynamicDataSourceType.Method)]
+        public void SubString(string input, long start, long? length, string expected)
+        {
+            input.FpSubstring(start, length).Should().Be(expected);
+        }
+
         [TestMethod]
         public void EncodeBase64()
         {