diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -8971,14 +8971,14 @@
         cast<TemplateTypeParmDecl>(TPL->getParam(0))->getTypeConstraint()
             ->getImmediatelyDeclaredConstraint();
     ExprResult Constraint = SubstExpr(IDC, MLTAL);
-    assert(!Constraint.isInvalid() &&
-           "Substitution cannot fail as it is simply putting a type template "
-           "argument into a concept specialization expression's parameter.");
-
-    SubstitutedConstraintExpr =
-        cast<ConceptSpecializationExpr>(Constraint.get());
-    if (!SubstitutedConstraintExpr->isSatisfied())
-      Status = concepts::ExprRequirement::SS_ConstraintsNotSatisfied;
+    if (Constraint.isInvalid()) {
+      Status = concepts::ExprRequirement::SS_ExprSubstitutionFailure;
+    } else {
+      SubstitutedConstraintExpr =
+          cast<ConceptSpecializationExpr>(Constraint.get());
+      if (!SubstitutedConstraintExpr->isSatisfied())
+        Status = concepts::ExprRequirement::SS_ConstraintsNotSatisfied;
+    }
   }
   return new (Context) concepts::ExprRequirement(E, IsSimple, NoexceptLoc,
                                                  ReturnTypeRequirement, Status,
diff --git a/clang/test/SemaCXX/concept-fatal-error.cpp b/clang/test/SemaCXX/concept-fatal-error.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaCXX/concept-fatal-error.cpp
@@ -0,0 +1,11 @@
+// RUN: not %clang_cc1 -fsyntax-only -std=c++20 -ferror-limit 1 %s 2>&1 | FileCheck %s
+
+template <class>
+concept f = requires { 42; };
+struct h {
+  // The missing semicolon will trigger an error and -ferror-limit=1 will make it fatal
+  // We test that we do not crash in such cases (#55401)
+  int i = requires { { i } f } // expected-error {{expected ';' at end of declaration list}}
+
+  // CHECK: fatal error: too many errors emitted, stopping now
+};