diff --git a/src/mgd77/cm4_functions.c b/src/mgd77/cm4_functions.c
index edcc135af81..0b72cb269bf 100644
--- a/src/mgd77/cm4_functions.c
+++ b/src/mgd77/cm4_functions.c
@@ -375,7 +375,6 @@ int MGD77_cm4field (struct GMT_CTRL *GMT, struct MGD77_CM4 *Ctrl, double *p_lon,
 		if (dstx) Ctrl->CM4_D.dst[0] = intdst(mjdl, mjdh, mjdy[0], msec[0], dstx, &cerr);
 		if (cerr > 49) {
 			gmt_M_str_free (dstx);
-			if (Ctrl->CM4_DATA.n_times > 1) gmt_M_str_free (Ctrl->CM4_D.dst);
 			clear_mem (mut, gpsq, gssq, gpmg, gsmg, hysq, epsq, essq, ecto, hyto, hq, ht, bkpo, ws, gamf, epmg,
 			           esmg, hymg, f107x, pleg, rcur, gcto_or, gcto_mg);
 			gmt_M_str_free (msec);	gmt_M_str_free (mjdy);	free (mjdy);
@@ -459,7 +458,7 @@ int MGD77_cm4field (struct GMT_CTRL *GMT, struct MGD77_CM4 *Ctrl, double *p_lon,
 		/* See if we are using a constant time or an array */
 		if (Ctrl->CM4_DATA.n_times > 1) {
 			date = p_date[n];
-			dst = Ctrl->CM4_D.dst[n];
+			dst = Ctrl->CM4_D.index ? Ctrl->CM4_D.dst[n] : Ctrl->CM4_D.dst[0];
 			mut_now = mut[n];
 		}
 		else {
diff --git a/src/mgd77/mgd77magref.c b/src/mgd77/mgd77magref.c
index e53fdd3896c..60fba28c226 100644
--- a/src/mgd77/mgd77magref.c
+++ b/src/mgd77/mgd77magref.c
@@ -82,6 +82,7 @@ static void Free_Ctrl (struct GMT_CTRL *GMT, struct MGD77MAGREF_CTRL *C) {	/* De
 	if (!C) return;
 	gmt_M_str_free (C->CM4->CM4_M.path);
 	gmt_M_str_free (C->CM4->CM4_D.path);
+	gmt_M_str_free (C->CM4->CM4_D.dst);
 	gmt_M_str_free (C->CM4->CM4_I.path);
 	gmt_M_str_free (C->CM4);
 	gmt_M_free (GMT, C);
@@ -237,10 +238,10 @@ static int parse (struct GMT_CTRL *GMT, struct MGD77MAGREF_CTRL *Ctrl, struct GM
 				break;
 			case 'D':
 				n_errors += gmt_M_repeated_module_option (API, Ctrl->D.active);
-				j = 0;
-				if (opt->arg[j] == '-') j++;
+				j = (opt->arg[0] == '-') ? 1 : 0;
 				if ((opt->arg[j] > 47) && (opt->arg[j] < 58)) {	/* arg is numeric -> Dst Index */
-					Ctrl->CM4->CM4_D.dst[0] = atof (&opt->arg[j]);
+					if (Ctrl->CM4->CM4_D.dst == NULL) Ctrl->CM4->CM4_D.dst = calloc (1U, sizeof(double));
+					Ctrl->CM4->CM4_D.dst[0] = atof (opt->arg);
 					Ctrl->CM4->CM4_D.index = false;
 				}
 				else {
@@ -494,7 +495,7 @@ EXTERN_MSC int GMT_mgd77magref (void *V_API, int mode, void *args) {
 
 	if (Ctrl->A.fixed_alt) t_col = 2;	/* Since we are missing the altitude column */
 
-	Ctrl->CM4->CM4_D.dst = calloc (1U, sizeof(double));	/* We need at least a size of one in case a value is given in input */
+	if (Ctrl->CM4->CM4_D.dst == NULL) Ctrl->CM4->CM4_D.dst = calloc (1U, sizeof(double));	/* We need at least a size of one in case a value is given in input */
 	if (!Ctrl->A.fixed_time)			/* Otherwise we don't print the time */
 		gmt_set_column_type (GMT, GMT_IO, t_col, GMT_IS_ABSTIME);
 
@@ -731,7 +732,6 @@ EXTERN_MSC int GMT_mgd77magref (void *V_API, int mode, void *args) {
 	}
 
 	gmt_M_free (GMT, Out);
-	gmt_M_str_free (Ctrl->CM4->CM4_D.dst);
 	gmt_M_free (GMT, Ctrl->CM4->CM4_DATA.out_field);
 	if (!(Ctrl->A.years || Ctrl->A.fixed_time)) gmt_M_free (GMT, time_years);
 	if (Ctrl->joint_IGRF_CM4) gmt_M_free (GMT, igrf_xyz);
diff --git a/test/mgd77/magref_dst.sh b/test/mgd77/magref_dst.sh
new file mode 100755
index 00000000000..1f16ab677f0
--- /dev/null
+++ b/test/mgd77/magref_dst.sh
@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+#
+# gmt mgd77magref -D<dst> with a numeric Dst value used to segfault (the value was stored
+# before its buffer was allocated) and dropped the sign of a negative value.
+
+rec="-30 45 0 2000.08700533"
+echo "$rec" | gmt mgd77magref -A+y -Fxyz/3 -D8   > pos.txt || exit 1
+echo "$rec" | gmt mgd77magref -A+y -Fxyz/3 -D-8  > neg.txt || exit 1
+# The sign must matter and the output must be finite
+cmp -s pos.txt neg.txt && { echo "-D-8 same as -D8: sign was dropped"; exit 1; }
+test -s pos.txt && test -s neg.txt || exit 1
+
+# With several records the single Dst value must apply to every row, not index past it
+printf -- "-30 45 0 2000.08700533\n-30 45 0 2000.5\n-30 45 0 2001.2\n" > multi.txt
+gmt mgd77magref multi.txt -A+y -Fxyz/3 -D8 > multi_all.txt || exit 1
+for i in 1 2 3; do
+	sed -n ${i}p multi.txt | gmt mgd77magref -A+y -Fxyz/3 -D8
+done > multi_one.txt || exit 1
+diff multi_all.txt multi_one.txt || exit 1
+
+# A segment rejected for lying outside the Dst table must not break the next segment
+printf -- "> bad\n-30 45 0 1900.5\n-30 45 0 1900.6\n> good\n-30 45 0 2000.5\n" > seg.txt
+gmt mgd77magref seg.txt -A+y -Fxyz/3 -Vq > seg_out.txt || exit 1
+grep -v '^>' seg_out.txt | wc -l | grep -q '^ *1$'