diff --git a/flang/lib/Optimizer/Builder/MutableBox.cpp b/flang/lib/Optimizer/Builder/MutableBox.cpp
--- a/flang/lib/Optimizer/Builder/MutableBox.cpp
+++ b/flang/lib/Optimizer/Builder/MutableBox.cpp
@@ -640,14 +640,17 @@
   auto idxTy = builder.getIndexType();
   if (auto charTy = box.getEleTy().dyn_cast<fir::CharacterType>()) {
     if (charTy.getLen() == fir::CharacterType::unknownLen()) {
-      if (box.hasNonDeferredLenParams())
+      if (box.hasNonDeferredLenParams()) {
         lengths.emplace_back(
             builder.createConvert(loc, idxTy, box.nonDeferredLenParams()[0]));
-      else if (!lenParams.empty())
-        lengths.emplace_back(builder.createConvert(loc, idxTy, lenParams[0]));
-      else
+      } else if (!lenParams.empty()) {
+        mlir::Value len =
+            fir::factory::genMaxWithZero(builder, loc, lenParams[0]);
+        lengths.emplace_back(builder.createConvert(loc, idxTy, len));
+      } else {
         fir::emitFatalError(
             loc, "could not deduce character lengths in character allocation");
+      }
     }
   }
   return lengths;
@@ -682,10 +685,13 @@
                                         mlir::ValueRange lenParams,
                                         llvm::StringRef allocName) {
   auto lengths = getNewLengths(builder, loc, box, lenParams);
+  llvm::SmallVector<mlir::Value> safeExtents;
+  for (mlir::Value extent : extents)
+    safeExtents.push_back(fir::factory::genMaxWithZero(builder, loc, extent));
   auto heap = builder.create<fir::AllocMemOp>(loc, box.getBaseTy(), allocName,
-                                              lengths, extents);
-  MutablePropertyWriter{builder, loc, box}.updateMutableBox(heap, lbounds,
-                                                            extents, lengths);
+                                              lengths, safeExtents);
+  MutablePropertyWriter{builder, loc, box}.updateMutableBox(
+      heap, lbounds, safeExtents, lengths);
   if (box.getEleTy().isa<fir::RecordType>()) {
     // TODO: skip runtime initialization if this is not required. Currently,
     // there is no way to know here if a derived type needs it or not. But the
diff --git a/flang/test/Lower/allocatables.f90 b/flang/test/Lower/allocatables.f90
--- a/flang/test/Lower/allocatables.f90
+++ b/flang/test/Lower/allocatables.f90
@@ -42,7 +42,9 @@
   ! CHECK-DAG: %[[c42:.*]] = fir.convert %c42{{.*}} : (i32) -> index
   ! CHECK-DAG: %[[c100:.*]] = fir.convert %c100_i32 : (i32) -> index
   ! CHECK-DAG: %[[diff:.*]] = arith.subi %[[c100]], %[[c42]] : index
-  ! CHECK: %[[extent:.*]] = arith.addi %[[diff]], %c1{{.*}} : index
+  ! CHECK: %[[rawExtent:.*]] = arith.addi %[[diff]], %c1{{.*}} : index
+  ! CHECK: %[[extentPositive:.*]] = arith.cmpi sgt, %[[rawExtent]], %c0{{.*}} : index
+  ! CHECK: %[[extent:.*]] = arith.select %[[extentPositive]], %[[rawExtent]], %c0{{.*}} : index
   ! CHECK: %[[alloc:.*]] = fir.allocmem !fir.array<?xf32>, %[[extent]] {{{.*}}uniq_name = "_QFfoodim1Ex.alloc"}
   ! CHECK-DAG: fir.store %[[alloc]] to %[[xAddrVar]] : !fir.ref<!fir.heap<!fir.array<?xf32>>>
   ! CHECK-DAG: fir.store %[[extent]] to %[[xExtVar]] : !fir.ref<index>
@@ -86,7 +88,9 @@
   ! CHECK: fir.freemem %{{.*}}
   allocate(character(n):: c)
   ! CHECK: %[[n:.*]] = fir.load %arg0 : !fir.ref<i32>
-  ! CHECK: %[[ni:.*]] = fir.convert %[[n]] : (i32) -> index
+  ! CHECK: %[[nPositive:.*]] = arith.cmpi sgt, %[[n]], %c0{{.*}} : i32
+  ! CHECK: %[[ns:.*]] = arith.select %[[nPositive]], %[[n]], %c0{{.*}} : i32
+  ! CHECK: %[[ni:.*]] = fir.convert %[[ns]] : (i32) -> index
   ! CHECK: fir.allocmem !fir.char<1,?>(%[[ni]] : index) {{{.*}}uniq_name = "_QFchar_deferredEc.alloc"}
   ! CHECK: fir.store %[[ni]] to %[[cLenVar]] : !fir.ref<index>