diff --git a/flang/lib/Evaluate/character.h b/flang/lib/Evaluate/character.h
--- a/flang/lib/Evaluate/character.h
+++ b/flang/lib/Evaluate/character.h
@@ -105,7 +105,7 @@
 
   static Character REPEAT(const Character &str, ConstantSubscript ncopies) {
     Character result;
-    if (!str.empty()) {
+    if (!str.empty() && ncopies > 0) {
       result.reserve(ncopies * str.size());
       while (ncopies-- > 0) {
         result += str;
diff --git a/flang/lib/Evaluate/fold-character.cpp b/flang/lib/Evaluate/fold-character.cpp
--- a/flang/lib/Evaluate/fold-character.cpp
+++ b/flang/lib/Evaluate/fold-character.cpp
@@ -99,7 +99,11 @@
             context, funcRef.arguments())}) {
       auto str{std::get<Scalar<T>>(*scalars)};
       auto n{std::get<Scalar<SubscriptInteger>>(*scalars).ToInt64()};
-      if (static_cast<double>(n) * str.size() >
+      if (n < 0) {
+        context.messages().Say(
+            "NCOPIES= argument to REPEAT() should be nonnegative, but is %jd"_err_en_US,
+            static_cast<std::intmax_t>(n));
+      } else if (static_cast<double>(n) * str.size() >
           (1 << 20)) { // sanity limit of 1MiB
         context.messages().Say(
             "Result of REPEAT() is too large to compute at compilation time (%g characters)"_port_en_US,
diff --git a/flang/test/Evaluate/errors01.f90 b/flang/test/Evaluate/errors01.f90
--- a/flang/test/Evaluate/errors01.f90
+++ b/flang/test/Evaluate/errors01.f90
@@ -148,6 +148,8 @@
   subroutine s13
     !CHECK: portability: Result of REPEAT() is too large to compute at compilation time (1.1259e+15 characters)
     print *, repeat(repeat(' ', 2**20), 2**30)
+    !CHECK: error: NCOPIES= argument to REPEAT() should be nonnegative, but is -666
+    print *, repeat(' ', -666)
   end subroutine
   subroutine warnings
     real, parameter :: ok1 = scale(0.0, 99999) ! 0.0