diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index c410ba532d2..0b5b77b0003 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -9950,7 +9950,14 @@ get_explain_guc_options(int *num)
 				{
 					struct config_string *lconf = (struct config_string *) conf;
 
-					modified = (strcmp(lconf->boot_val, *(lconf->variable)) != 0);
+					if (lconf->boot_val == NULL &&
+						*lconf->variable == NULL)
+						modified = false;
+					else if (lconf->boot_val == NULL ||
+							 *lconf->variable == NULL)
+						modified = true;
+					else
+						modified = (strcmp(lconf->boot_val, *(lconf->variable)) != 0);
 				}
 				break;
 
@@ -10694,7 +10701,8 @@ write_one_nondefault_variable(FILE *fp, struct config_generic *gconf)
 			{
 				struct config_string *conf = (struct config_string *) gconf;
 
-				fprintf(fp, "%s", *conf->variable);
+				if (*conf->variable)
+					fprintf(fp, "%s", *conf->variable);
 			}
 			break;
 
diff --git a/src/include/utils/guc_tables.h b/src/include/utils/guc_tables.h
index 067d82ada95..47e7819018e 100644
--- a/src/include/utils/guc_tables.h
+++ b/src/include/utils/guc_tables.h
@@ -227,6 +227,16 @@ struct config_real
 	void	   *reset_extra;
 };
 
+/*
+ * A note about string GUCs: the boot_val is allowed to be NULL, which leads
+ * to the reset_val and the actual variable value (*variable) also being NULL.
+ * However, there is no way to set a NULL value subsequently using
+ * set_config_option or any other GUC API.  Also, GUC APIs such as SHOW will
+ * display a NULL value as an empty string.  Callers that choose to use a NULL
+ * boot_val should overwrite the setting later in startup, or else be careful
+ * that NULL doesn't have semantics that are visibly different from an empty
+ * string.
+ */
 struct config_string
 {
 	struct config_generic gen;