diff --git a/FuzzySearchNet/src/FuzzySearchExact.cs b/FuzzySearchNet/src/FuzzySearchExact.cs
index 93172b4..4c09cbe 100644
--- a/FuzzySearchNet/src/FuzzySearchExact.cs
+++ b/FuzzySearchNet/src/FuzzySearchExact.cs
@@ -14,7 +14,7 @@ public static IEnumerable<MatchResult> FindExact(string subSequence, string text
             yield break;
         }
 
-        subSequence = invariantCultureIgnoreCase ? subSequence.ToLowerInvariant() : subSequence;
+        subSequence = invariantCultureIgnoreCase ? subSequence.ToUpperInvariant() : subSequence;
 
         // indexof would probably run circles around this...
         var needlePosition = 0;
@@ -23,7 +23,7 @@ public static IEnumerable<MatchResult> FindExact(string subSequence, string text
 
         foreach (var currentCharacter in text)
         {
-            if ((invariantCultureIgnoreCase ? char.ToLowerInvariant(currentCharacter) : currentCharacter) == subSequence[needlePosition])
+            if ((invariantCultureIgnoreCase ? char.ToUpperInvariant(currentCharacter) : currentCharacter) == subSequence[needlePosition])
             {
                 if (needlePosition == termLength)
                 {
@@ -67,7 +67,7 @@ public static async IAsyncEnumerable<MatchResult> FindExactAsync(string subSeque
             yield break;
         }
 
-        subSequence = invariantCultureIgnoreCase ? subSequence.ToLowerInvariant() : subSequence;
+        subSequence = invariantCultureIgnoreCase ? subSequence.ToUpperInvariant() : subSequence;
 
         var needlePosition = 0;
         var termLength = subSequence.Length - 1;
@@ -83,7 +83,7 @@ public static async IAsyncEnumerable<MatchResult> FindExactAsync(string subSeque
         {
             for (var currentIndex = 0; currentIndex < bytesRead; currentIndex++)
             {
-                if ((invariantCultureIgnoreCase ? char.ToLowerInvariant(buffer[currentIndex]) : buffer[currentIndex]) == subSequence[needlePosition])
+                if ((invariantCultureIgnoreCase ? char.ToUpperInvariant(buffer[currentIndex]) : buffer[currentIndex]) == subSequence[needlePosition])
                 {
                     if (needlePosition == termLength)
                     {
diff --git a/FuzzySearchNet/src/FuzzySearchLevenshtein.cs b/FuzzySearchNet/src/FuzzySearchLevenshtein.cs
index 32d4be8..fc94ee5 100644
--- a/FuzzySearchNet/src/FuzzySearchLevenshtein.cs
+++ b/FuzzySearchNet/src/FuzzySearchLevenshtein.cs
@@ -24,7 +24,7 @@ internal static IEnumerable<CandidateMatch> FindLevenshteinAll(string subSequenc
             yield break;
         }
 
-        subSequence = options.InvariantCultureIgnoreCase ? subSequence.ToLowerInvariant() : subSequence;
+        subSequence = options.InvariantCultureIgnoreCase ? subSequence.ToUpperInvariant() : subSequence;
         var candidates = new Stack<CandidateMatch>();
 
         for (var currentIndex = 0; currentIndex < text.Length; currentIndex++)
@@ -79,7 +79,7 @@ internal static async IAsyncEnumerable<MatchResultWithValue> FindLevenshteinAllA
         }
 
         var candidates = new Stack<CandidateMatch>();
-        subSequence = options.InvariantCultureIgnoreCase ? subSequence.ToLowerInvariant() : subSequence;
+        subSequence = options.InvariantCultureIgnoreCase ? subSequence.ToUpperInvariant() : subSequence;
 
         var startBuffer = subSequence.Length + options.MaxTotalDistance;
         bufferSize = ((startBuffer * 2 / bufferSize) + 1) * bufferSize;
@@ -146,7 +146,7 @@ internal static async IAsyncEnumerable<MatchResultWithValue> FindLevenshteinAllA
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     internal static void HandleCandidate(Stack<CandidateMatch> candidates, in CandidateMatch candidate, ReadOnlySpan<char> text, string subSequence, int bestFoundDistance, FuzzySearchOptions options, int textLength)
     {
-        if (candidate.TextIndex < textLength && (options.InvariantCultureIgnoreCase ? char.ToLowerInvariant(text[candidate.TextIndex]) : text[candidate.TextIndex]) == subSequence[candidate.SubSequenceIndex])
+        if (candidate.TextIndex < textLength && (options.InvariantCultureIgnoreCase ? char.ToUpperInvariant(text[candidate.TextIndex]) : text[candidate.TextIndex]) == subSequence[candidate.SubSequenceIndex])
         {
             if (candidate.Distance < bestFoundDistance && options.CanInsert(candidate.Distance, candidate.Insertions))
             {
diff --git a/FuzzySearchNet/src/FuzzySearchSubstitutionsOnly.cs b/FuzzySearchNet/src/FuzzySearchSubstitutionsOnly.cs
index f5ea172..15bd969 100644
--- a/FuzzySearchNet/src/FuzzySearchSubstitutionsOnly.cs
+++ b/FuzzySearchNet/src/FuzzySearchSubstitutionsOnly.cs
@@ -12,7 +12,7 @@ public static IEnumerable<MatchResult> FindSubstitutionsOnly(string subSequence,
             yield break;
         }
 
-        subSequence = invariantCultureIgnoreCase ? subSequence.ToLowerInvariant() : subSequence;
+        subSequence = invariantCultureIgnoreCase ? subSequence.ToUpperInvariant() : subSequence;
 
         for (var currentIndex = 0; currentIndex < text.Length - (subSequence.Length - 1); currentIndex++)
         {
@@ -21,7 +21,7 @@ public static IEnumerable<MatchResult> FindSubstitutionsOnly(string subSequence,
 
             for (var termIndex = 0; termIndex < subSequence.Length; termIndex++)
             {
-                if ((invariantCultureIgnoreCase ? char.ToLowerInvariant(text[needlePosition]) : text[needlePosition]) != subSequence[termIndex])
+                if ((invariantCultureIgnoreCase ? char.ToUpperInvariant(text[needlePosition]) : text[needlePosition]) != subSequence[termIndex])
                 {
                     candidateDistance++;
                     if (candidateDistance > maxDistance)
@@ -61,7 +61,7 @@ public static async IAsyncEnumerable<MatchResult> FindSubstitutionsOnlyAsync(str
             yield break;
         }
 
-        subSequence = invariantCultureIgnoreCase ? subSequence.ToLowerInvariant() : subSequence;
+        subSequence = invariantCultureIgnoreCase ? subSequence.ToUpperInvariant() : subSequence;
 
         var subSequenceLengthMinusOne = subSequence.Length - 1;
 
@@ -89,7 +89,7 @@ public static async IAsyncEnumerable<MatchResult> FindSubstitutionsOnlyAsync(str
                 for (var subSequenceIndex = 0; subSequenceIndex < subSequence.Length; subSequenceIndex++)
                 {
                     var match = invariantCultureIgnoreCase
-                        ? char.ToLowerInvariant(buffer[needlePosition]) == subSequence[subSequenceIndex]
+                        ? char.ToUpperInvariant(buffer[needlePosition]) == subSequence[subSequenceIndex]
                         : buffer[needlePosition] == subSequence[subSequenceIndex];
 
                     if (!match)