diff --git a/flang/include/flang/Optimizer/Builder/Character.h b/flang/include/flang/Optimizer/Builder/Character.h
--- a/flang/include/flang/Optimizer/Builder/Character.h
+++ b/flang/include/flang/Optimizer/Builder/Character.h
@@ -204,8 +204,8 @@
 mlir::Type getCharacterProcedureTupleType(mlir::Type funcPointerType);
 
 /// Create a tuple<addr, len> given \p addr and \p len as well as the tuple
-/// type \p argTy. \p addr must be any function address, and \p len must be
-/// any integer. Converts will be inserted if needed if \addr and \p len
+/// type \p argTy. \p addr must be any function address, and \p len may be any
+/// integer or nullptr. Converts will be inserted if needed if \addr and \p len
 /// types are not the same as the one inside the tuple type \p tupleType.
 mlir::Value createCharacterProcedureTuple(fir::FirOpBuilder &builder,
                                           mlir::Location loc,
diff --git a/flang/lib/Optimizer/Builder/Character.cpp b/flang/lib/Optimizer/Builder/Character.cpp
--- a/flang/lib/Optimizer/Builder/Character.cpp
+++ b/flang/lib/Optimizer/Builder/Character.cpp
@@ -718,7 +718,10 @@
     mlir::Value addr, mlir::Value len) {
   mlir::TupleType tupleType = argTy.cast<mlir::TupleType>();
   addr = builder.createConvert(loc, tupleType.getType(0), addr);
-  len = builder.createConvert(loc, tupleType.getType(1), len);
+  if (len)
+    len = builder.createConvert(loc, tupleType.getType(1), len);
+  else
+    len = builder.create<fir::UndefOp>(loc, tupleType.getType(1));
   mlir::Value tuple = builder.create<fir::UndefOp>(loc, tupleType);
   tuple = builder.create<fir::InsertValueOp>(
       loc, tupleType, tuple, addr,
diff --git a/flang/test/Lower/ext-proc-as-actual-argument-1.f90 b/flang/test/Lower/ext-proc-as-actual-argument-1.f90
new file mode 100644
--- /dev/null
+++ b/flang/test/Lower/ext-proc-as-actual-argument-1.f90
@@ -0,0 +1,31 @@
+! RUN: bbc -emit-fir %s -o - | FileCheck %s
+
+! Test external procedure as actual argument with the implicit character type.
+
+! CHECK-LABEL: func @_QQmain
+! CHECK:  %[[VAL_0:.*]] = fir.address_of(@_QPext_func) : (!fir.ref<!fir.char<1,?>>, index) -> !fir.boxchar<1>
+! CHECK:  %[[VAL_1:.*]] = fir.emboxproc %[[VAL_0]] : ((!fir.ref<!fir.char<1,?>>, index) -> !fir.boxchar<1>) -> !fir.boxproc<() -> ()>
+! CHECK:  %[[VAL_2:.*]] = fir.undefined i64
+! CHECK:  %[[VAL_3:.*]] = fir.undefined tuple<!fir.boxproc<() -> ()>, i64>
+! CHECK:  %[[VAL_4:.*]] = fir.insert_value %[[VAL_3]], %[[VAL_1]], [0 : index] : (tuple<!fir.boxproc<() -> ()>, i64>, !fir.boxproc<() -> ()>) -> tuple<!fir.boxproc<() -> ()>, i64>
+! CHECK:  %[[VAL_5:.*]] = fir.insert_value %[[VAL_4]], %[[VAL_2]], [1 : index] : (tuple<!fir.boxproc<() -> ()>, i64>, i64) -> tuple<!fir.boxproc<() -> ()>, i64>
+! CHECK:  fir.call @_QFPsub(%[[VAL_5]]) : (tuple<!fir.boxproc<() -> ()>, i64>) -> ()
+! CHECK:  return
+
+! CHECK-LABEL: func @_QPext_func(
+! CEHCK: %[[ARG_0:.*]]: !fir.ref<!fir.char<1,?>>, %[[ARG_1:.*]]: index) -> !fir.boxchar<1> {
+program m
+  external :: ext_func
+  call sub(ext_func)
+
+contains
+  subroutine sub(arg)
+    character(20), external :: arg
+    print *, arg()
+  end
+end
+
+function ext_func() result(res)
+  character(*) res
+  res = "hello world"
+end
diff --git a/flang/test/Lower/ext-proc-as-actual-argument-2.f90 b/flang/test/Lower/ext-proc-as-actual-argument-2.f90
new file mode 100644
--- /dev/null
+++ b/flang/test/Lower/ext-proc-as-actual-argument-2.f90
@@ -0,0 +1,31 @@
+! RUN: bbc -emit-fir %s -o - | FileCheck %s
+
+! Test external procedure as actual argument with the implicit character type.
+
+! CHECK-LABEL: func @_QQmain
+! CHECK:  %[[VAL_0:.*]] = fir.address_of(@_QPext_func) : (!fir.ref<!fir.char<1,20>>, index) -> !fir.boxchar<1>
+! CHECK:  %[[VAL_1:.*]] = fir.emboxproc %[[VAL_0]] : ((!fir.ref<!fir.char<1,20>>, index) -> !fir.boxchar<1>) -> !fir.boxproc<() -> ()>
+! CHECK:  %[[VAL_2:.*]] = fir.undefined i64
+! CHECK:  %[[VAL_3:.*]] = fir.undefined tuple<!fir.boxproc<() -> ()>, i64>
+! CHECK:  %[[VAL_4:.*]] = fir.insert_value %[[VAL_3]], %[[VAL_1]], [0 : index] : (tuple<!fir.boxproc<() -> ()>, i64>, !fir.boxproc<() -> ()>) -> tuple<!fir.boxproc<() -> ()>, i64>
+! CHECK:  %[[VAL_5:.*]] = fir.insert_value %[[VAL_4]], %[[VAL_2]], [1 : index] : (tuple<!fir.boxproc<() -> ()>, i64>, i64) -> tuple<!fir.boxproc<() -> ()>, i64>
+! CHECK:  fir.call @_QFPsub(%[[VAL_5]]) : (tuple<!fir.boxproc<() -> ()>, i64>) -> ()
+! CHECK:  return
+
+! CHECK-LABEL: func @_QPext_func(
+! CEHCK: %[[ARG_0:.*]]: !fir.ref<!fir.char<1,20>>, %[[ARG_1:.*]]: index) -> !fir.boxchar<1> {
+program m
+  external :: ext_func
+  call sub(ext_func)
+
+contains
+  subroutine sub(arg)
+    character(20), external :: arg
+    print *, arg()
+  end
+end
+
+function ext_func() result(res)
+  character(20) res
+  res = "hello world"
+end