diff --git a/ketopt.h b/ketopt.h
index 70193a5..8ae1811 100644
--- a/ketopt.h
+++ b/ketopt.h
@@ -73,13 +73,17 @@ static int ketopt(ketopt_t *s, int argc, char *argv[], int permute, const char *
 		}
 		s->opt = 0, opt = '?', s->pos = -1;
 		if (longopts) { /* parse long options */
-			int k, n_matches = 0;
-			const ko_longopt_t *o = 0;
+			int k, n_exact = 0, n_partial = 0;
+			const ko_longopt_t *o = 0, *o_exact = 0, *o_partial = 0;
 			for (j = 2; argv[s->i][j] != '\0' && argv[s->i][j] != '='; ++j) {} /* find the end of the option name */
 			for (k = 0; longopts[k].name != 0; ++k)
-				if (strncmp(&argv[s->i][2], longopts[k].name, j - 2) == 0)
-					++n_matches, o = &longopts[k];
-			if (n_matches == 1) {
+				if (strncmp(&argv[s->i][2], longopts[k].name, j - 2) == 0) {
+					if (longopts[k].name[j - 2] == 0) ++n_exact, o_exact = &longopts[k];
+					else ++n_partial, o_partial = &longopts[k];
+				}
+			if (n_exact > 1 || (n_exact == 0 && n_partial > 1)) return '?';
+			o = n_exact == 1? o_exact : n_partial == 1? o_partial : 0;
+			if (o) {
 				s->opt = opt = o->val, s->longidx = o - longopts;
 				if (argv[s->i][j] == '=') s->arg = &argv[s->i][j + 1];
 				if (o->has_arg == 1 && argv[s->i][j] == '\0') {
diff --git a/main.c b/main.c
index fac05b1..34c4696 100644
--- a/main.c
+++ b/main.c
@@ -6,7 +6,7 @@
 #include "mmpriv.h"
 #include "ketopt.h"
 
-#define MM_VERSION "2.15-r915-dirty"
+#define MM_VERSION "2.15-r917-dirty"
 
 #ifdef __linux__
 #include <sys/resource.h>