diff --git a/stdlib/public/core/Assert.swift b/stdlib/public/core/Assert.swift
index f45a24e2056..4b1c847e76a 100644
--- a/stdlib/public/core/Assert.swift
+++ b/stdlib/public/core/Assert.swift
@@ -236,7 +236,7 @@ public func _overflowChecked<T>(
   let (result, error) = args
   if _isDebugAssertConfiguration() {
     if _slowPath(error) {
-      _fatalErrorMessage("Fatal error", "Overflow/underflow", 
+      _fatalErrorMessage("Fatal error", "Overflow/underflow",
         file: file, line: line, flags: _fatalErrorFlags())
     }
   } else {
@@ -297,6 +297,22 @@ internal func _internalInvariant(
 #endif
 }
 
+// Only perform the invariant check on Swift 5.1 and later
+@_alwaysEmitIntoClient // Swift 5.1
+@_transparent
+internal func _internalInvariant_5_1(
+  _ condition: @autoclosure () -> Bool, _ message: StaticString = StaticString(),
+  file: StaticString = #file, line: UInt = #line
+) {
+#if INTERNAL_CHECKS_ENABLED
+  // FIXME: The below won't run the assert on 5.1 stdlib if testing on older
+  // OSes, which means that testing may not test the assertion. We need a real
+  // solution to this.
+  guard #available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *) else { return }
+  _internalInvariant(condition(), message, file: file, line: line)
+#endif
+}
+
 @usableFromInline @_transparent
 internal func _internalInvariantFailure(
   _ message: StaticString = StaticString(),
diff --git a/stdlib/public/core/StringCharacterView.swift b/stdlib/public/core/StringCharacterView.swift
index d1cfeb44b7c..236bb06e9c8 100644
--- a/stdlib/public/core/StringCharacterView.swift
+++ b/stdlib/public/core/StringCharacterView.swift
@@ -201,7 +201,7 @@ extension String: BidirectionalCollection {
 
   @inlinable @inline(__always)
   internal func _characterStride(startingAt i: Index) -> Int {
-    _internalInvariant(i._isScalarAligned)
+    _internalInvariant_5_1(i._isScalarAligned)
 
     // Fast check if it's already been measured, otherwise check resiliently
     if let d = i.characterStride { return d }
@@ -213,7 +213,7 @@ extension String: BidirectionalCollection {
 
   @inlinable @inline(__always)
   internal func _characterStride(endingAt i: Index) -> Int {
-    _internalInvariant(i._isScalarAligned)
+    _internalInvariant_5_1(i._isScalarAligned)
 
     if i == startIndex { return 0 }
 
diff --git a/stdlib/public/core/StringIndex.swift b/stdlib/public/core/StringIndex.swift
index 24d34387aa2..7d8ec695113 100644
--- a/stdlib/public/core/StringIndex.swift
+++ b/stdlib/public/core/StringIndex.swift
@@ -164,7 +164,7 @@ extension String.Index {
   internal func _invariantCheck() {
     _internalInvariant(_encodedOffset >= 0)
     if self._isScalarAligned {
-      _internalInvariant(transcodedOffset == 0)
+      _internalInvariant_5_1(transcodedOffset == 0)
     }
   }
   #endif // INTERNAL_CHECKS_ENABLED
diff --git a/stdlib/public/core/UnicodeHelpers.swift b/stdlib/public/core/UnicodeHelpers.swift
index 339dfcf439a..b8e0d57cbd4 100644
--- a/stdlib/public/core/UnicodeHelpers.swift
+++ b/stdlib/public/core/UnicodeHelpers.swift
@@ -172,14 +172,14 @@ extension _StringGuts {
 
     _internalInvariant(isOnUnicodeScalarBoundary(result),
       "Alignment bit is set for non-aligned index")
-    _internalInvariant(result._isScalarAligned)
+    _internalInvariant_5_1(result._isScalarAligned)
     return result
   }
 
   @inline(never) // slow-path
   @_alwaysEmitIntoClient // Swift 5.1
   internal func scalarAlignSlow(_ idx: Index) -> Index {
-    _internalInvariant(!idx._isScalarAligned)
+    _internalInvariant_5_1(!idx._isScalarAligned)
 
     if _slowPath(idx.transcodedOffset != 0 || idx._encodedOffset == 0) {
       // Transcoded index offsets are already scalar aligned
@@ -187,7 +187,7 @@ extension _StringGuts {
     }
     if _slowPath(self.isForeign) {
       let foreignIdx = foreignScalarAlign(idx)
-      _internalInvariant(foreignIdx._isScalarAligned)
+      _internalInvariant_5_1(foreignIdx._isScalarAligned)
       return foreignIdx
     }