diff --git a/src/backend/executor/execExpr.c b/src/backend/executor/execExpr.c
index ab89ad9afbf..1e3b93a69d8 100644
--- a/src/backend/executor/execExpr.c
+++ b/src/backend/executor/execExpr.c
@@ -1188,6 +1188,14 @@ ExecInitExprRec(Expr *node, ExprState *state,
 							 op->args, op->opfuncid, op->inputcollid,
 							 state);
 
+				/*
+				 * If first argument is of varlena type, we'll need to ensure
+				 * that the value passed to the comparison function is a
+				 * read-only pointer.
+				 */
+				scratch.d.func.make_ro =
+					(get_typlen(exprType((Node *) linitial(op->args))) == -1);
+
 				/*
 				 * Change opcode of call instruction to EEOP_NULLIF.
 				 *
diff --git a/src/backend/executor/execExprInterp.c b/src/backend/executor/execExprInterp.c
index aa68c115ba9..a55ad772566 100644
--- a/src/backend/executor/execExprInterp.c
+++ b/src/backend/executor/execExprInterp.c
@@ -1295,12 +1295,24 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 			 * The arguments are already evaluated into fcinfo->args.
 			 */
 			FunctionCallInfo fcinfo = op->d.func.fcinfo_data;
+			Datum		save_arg0 = fcinfo->args[0].value;
 
 			/* if either argument is NULL they can't be equal */
 			if (!fcinfo->args[0].isnull && !fcinfo->args[1].isnull)
 			{
 				Datum		result;
 
+				/*
+				 * If first argument is of varlena type, it might be an
+				 * expanded datum.  We need to ensure that the value passed to
+				 * the comparison function is a read-only pointer.  However,
+				 * if we end by returning the first argument, that will be the
+				 * original read-write pointer if it was read-write.
+				 */
+				if (op->d.func.make_ro)
+					fcinfo->args[0].value =
+						MakeExpandedObjectReadOnlyInternal(save_arg0);
+
 				fcinfo->isnull = false;
 				result = op->d.func.fn_addr(fcinfo);
 
@@ -1315,7 +1327,7 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 			}
 
 			/* Arguments aren't equal, so return the first one */
-			*op->resvalue = fcinfo->args[0].value;
+			*op->resvalue = save_arg0;
 			*op->resnull = fcinfo->args[0].isnull;
 
 			EEO_NEXT();
diff --git a/src/backend/jit/llvm/llvmjit_expr.c b/src/backend/jit/llvm/llvmjit_expr.c
index 306aea82d3b..b9fdd444a25 100644
--- a/src/backend/jit/llvm/llvmjit_expr.c
+++ b/src/backend/jit/llvm/llvmjit_expr.c
@@ -1562,6 +1562,9 @@ llvm_compile_expr(ExprState *state)
 
 					v_fcinfo = l_ptr_const(fcinfo, l_ptr(StructFunctionCallInfoData));
 
+					/* save original arg[0] */
+					v_arg0 = l_funcvalue(b, v_fcinfo, 0);
+
 					/* if either argument is NULL they can't be equal */
 					v_argnull0 = l_funcnull(b, v_fcinfo, 0);
 					v_argnull1 = l_funcnull(b, v_fcinfo, 1);
@@ -1578,7 +1581,6 @@ llvm_compile_expr(ExprState *state)
 
 					/* one (or both) of the arguments are null, return arg[0] */
 					LLVMPositionBuilderAtEnd(b, b_hasnull);
-					v_arg0 = l_funcvalue(b, v_fcinfo, 0);
 					LLVMBuildStore(b, v_argnull0, v_resnullp);
 					LLVMBuildStore(b, v_arg0, v_resvaluep);
 					LLVMBuildBr(b, opblocks[opno + 1]);
@@ -1586,12 +1588,35 @@ llvm_compile_expr(ExprState *state)
 					/* build block to invoke function and check result */
 					LLVMPositionBuilderAtEnd(b, b_nonull);
 
+					/*
+					 * If first argument is of varlena type, it might be an
+					 * expanded datum.  We need to ensure that the value
+					 * passed to the comparison function is a read-only
+					 * pointer.  However, if we end by returning the first
+					 * argument, that will be the original read-write pointer
+					 * if it was read-write.
+					 */
+					if (op->d.func.make_ro)
+					{
+						LLVMValueRef v_params[1];
+						LLVMValueRef v_arg0_ro;
+
+						v_params[0] = v_arg0;
+						v_arg0_ro =
+							l_call(b,
+								   llvm_pg_var_func_type("MakeExpandedObjectReadOnlyInternal"),
+								   llvm_pg_func(mod, "MakeExpandedObjectReadOnlyInternal"),
+								   v_params, lengthof(v_params), "");
+						LLVMBuildStore(b, v_arg0_ro,
+									   l_funcvaluep(b, v_fcinfo, 0));
+					}
+
 					v_retval = BuildV1Call(context, b, mod, fcinfo, &v_fcinfo_isnull);
 
 					/*
-					 * If result not null, and arguments are equal return null
-					 * (same result as if there'd been NULLs, hence reuse
-					 * b_hasnull).
+					 * If result not null and arguments are equal return null,
+					 * else return arg[0] (same result as if there'd been
+					 * NULLs, hence reuse b_hasnull).
 					 */
 					v_argsequal = LLVMBuildAnd(b,
 											   LLVMBuildICmp(b, LLVMIntEQ,
diff --git a/src/include/executor/execExpr.h b/src/include/executor/execExpr.h
index d983a9a7fed..ef1fa37716d 100644
--- a/src/include/executor/execExpr.h
+++ b/src/include/executor/execExpr.h
@@ -356,6 +356,7 @@ typedef struct ExprEvalStep
 			/* faster to access without additional indirection: */
 			PGFunction	fn_addr;	/* actual call address */
 			int			nargs;	/* number of arguments */
+			bool		make_ro;	/* make arg0 R/O (used only for NULLIF) */
 		}			func;
 
 		/* for EEOP_BOOL_*_STEP */
diff --git a/src/test/regress/expected/case.out b/src/test/regress/expected/case.out
index f5136c17abb..efee7fc4317 100644
--- a/src/test/regress/expected/case.out
+++ b/src/test/regress/expected/case.out
@@ -397,6 +397,14 @@ SELECT CASE make_ad(1,2)
  right
 (1 row)
 
+-- While we're here, also test handling of a NULLIF arg that is a read/write
+-- object (bug #18722)
+SELECT NULLIF(make_ad(1,2), array[2,3]::arrdomain);
+ nullif 
+--------
+ {1,2}
+(1 row)
+
 ROLLBACK;
 -- Test interaction of CASE with ArrayCoerceExpr (bug #15471)
 BEGIN;
diff --git a/src/test/regress/sql/case.sql b/src/test/regress/sql/case.sql
index 83fe43be6b8..388d4c6f528 100644
--- a/src/test/regress/sql/case.sql
+++ b/src/test/regress/sql/case.sql
@@ -242,6 +242,11 @@ SELECT CASE make_ad(1,2)
   WHEN array[1,2]::arrdomain THEN 'right'
   END;
 
+-- While we're here, also test handling of a NULLIF arg that is a read/write
+-- object (bug #18722)
+
+SELECT NULLIF(make_ad(1,2), array[2,3]::arrdomain);
+
 ROLLBACK;
 
 -- Test interaction of CASE with ArrayCoerceExpr (bug #15471)