diff --git a/src/backend/executor/execExpr.c b/src/backend/executor/execExpr.c
index 8f7a534005..69d36f70b3 100644
--- a/src/backend/executor/execExpr.c
+++ b/src/backend/executor/execExpr.c
@@ -1191,6 +1191,14 @@ 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 30c5a19aad..42da962178 100644
--- a/src/backend/executor/execExprInterp.c
+++ b/src/backend/executor/execExprInterp.c
@@ -1308,12 +1308,24 @@ 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);
 
@@ -1328,7 +1340,7 @@ 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/include/executor/execExpr.h b/src/include/executor/execExpr.h
index cd97dfa062..56fb0d0adb 100644
--- a/src/include/executor/execExpr.h
+++ b/src/include/executor/execExpr.h
@@ -365,6 +365,7 @@ 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 */