diff --git a/flang/lib/Optimizer/Dialect/FIRType.cpp b/flang/lib/Optimizer/Dialect/FIRType.cpp
--- a/flang/lib/Optimizer/Dialect/FIRType.cpp
+++ b/flang/lib/Optimizer/Dialect/FIRType.cpp
@@ -518,8 +518,12 @@
         name << "x" << charTy.getLen();
       break;
     } else if (auto seqTy = mlir::dyn_cast_or_null<fir::SequenceType>(ty)) {
-      for (auto extent : seqTy.getShape())
-        name << extent << 'x';
+      for (auto extent : seqTy.getShape()) {
+        if (extent == fir::SequenceType::getUnknownExtent())
+          name << "?x";
+        else
+          name << extent << 'x';
+      }
       ty = seqTy.getEleTy();
     } else if (auto refTy = mlir::dyn_cast_or_null<fir::ReferenceType>(ty)) {
       name << "ref_";
diff --git a/flang/unittests/Optimizer/FIRTypesTest.cpp b/flang/unittests/Optimizer/FIRTypesTest.cpp
--- a/flang/unittests/Optimizer/FIRTypesTest.cpp
+++ b/flang/unittests/Optimizer/FIRTypesTest.cpp
@@ -304,4 +304,9 @@
   components.emplace_back("p1", mlir::IntegerType::get(&context, 64));
   derivedTy.finalize({}, components);
   EXPECT_EQ("rec_derived", fir::getTypeAsString(derivedTy, *kindMap));
+  mlir::Type dynArrTy =
+      fir::SequenceType::get({fir::SequenceType::getUnknownExtent(),
+                                 fir::SequenceType::getUnknownExtent()},
+          ty);
+  EXPECT_EQ("?x?xi64", fir::getTypeAsString(dynArrTy, *kindMap));
 }