diff --git a/libavutil/opt.c b/libavutil/opt.c
index 0f69e04b3b..45c9105b90 100644
--- a/libavutil/opt.c
+++ b/libavutil/opt.c
@@ -505,11 +505,20 @@ static int set_string_number(void *obj, void *target_obj, const AVOption *o, con
             }
         }
         if (type == AV_OPT_TYPE_FLAGS && dst) {
+            unsigned int operand;
+            /* Validate the operand of a '+' or '-' update and let
+             * write_number() convert it: casting a double outside the
+             * int64_t range (or inf/nan) to int64_t is undefined behaviour,
+             * and the cast would also drop a fractional part that
+             * write_number() is meant to reject, so "+1.5" used to be
+             * accepted while "1.5" is refused. */
+            if (cmd && (ret = write_number(obj, o, &operand, d, 1, 1, 0)) < 0)
+                return ret;
             intnum = *(unsigned int*)dst;
             if (cmd == '+')
-                d = intnum | (int64_t)d;
+                d = intnum | operand;
             else if (cmd == '-')
-                d = intnum &~(int64_t)d;
+                d = intnum &~operand;
         }
 
         if ((ret = write_number(obj, o, dst, d, 1, 1, 0)) < 0)
diff --git a/libavutil/tests/opt.c b/libavutil/tests/opt.c
index 1f35d3dcb7..55ec7100c1 100644
--- a/libavutil/tests/opt.c
+++ b/libavutil/tests/opt.c
@@ -479,6 +479,10 @@ int main(void)
             "toggle=100",
             "toggle==1",
             "flags=+mu-lame : num=42: toggle=0",
+            "flags=+1.5",
+            "flags=-1.5",
+            "flags=+100000000000000000000",
+            "flags=-100000000000000000000",
             "num=42 : string=blahblah",
             "rational=0 : rational=1/2 : rational=1/-1",
             "rational=-1/0",
@@ -549,7 +553,9 @@ int main(void)
         av_log_set_level(AV_LOG_QUIET);
 
         for (i=0; i < FF_ARRAY_ELEMS(options); i++) {
-            int silence_log = !strcmp(options[i], "rational=-1/0"); // inf formatting differs between platforms
+            // inf formatting differs between platforms, and so may %f of 1e20
+            int silence_log = !strcmp(options[i], "rational=-1/0") ||
+                              strstr(options[i], "100000000000000000000");
             av_log(&test_ctx, AV_LOG_DEBUG, "Setting options string '%s'\n", options[i]);
             if (silence_log)
                 av_log_set_callback(NULL);
@@ -559,6 +565,8 @@ int main(void)
                 printf("OK    '%s'\n", options[i]);
             av_log_set_callback(log_callback_help);
         }
+        // a rejected +x/-x flags update must leave the value untouched
+        printf("flags=%d\n", test_ctx.flags);
         av_opt_free(&test_ctx);
     }
 
diff --git a/tests/ref/fate/opt b/tests/ref/fate/opt
index 90a7ba85e9..e2be9f9663 100644
--- a/tests/ref/fate/opt
+++ b/tests/ref/fate/opt
@@ -269,6 +269,18 @@ Setting entry with key 'flags' to value '+mu-lame'
 Setting entry with key 'num' to value '42'
 Setting entry with key 'toggle' to value '0'
 OK    'flags=+mu-lame : num=42: toggle=0'
+Setting options string 'flags=+1.5'
+Setting entry with key 'flags' to value '+1.5'
+Value 1.500000 for parameter 'flags' is not a valid set of 32bit integer flags
+Error 'flags=+1.5'
+Setting options string 'flags=-1.5'
+Setting entry with key 'flags' to value '-1.5'
+Value 1.500000 for parameter 'flags' is not a valid set of 32bit integer flags
+Error 'flags=-1.5'
+Setting options string 'flags=+100000000000000000000'
+Error 'flags=+100000000000000000000'
+Setting options string 'flags=-100000000000000000000'
+Error 'flags=-100000000000000000000'
 Setting options string 'num=42 : string=blahblah'
 Setting entry with key 'num' to value '42'
 Setting entry with key 'string' to value 'blahblah'
@@ -487,6 +499,7 @@ Setting options string 'array_int=2147483648'
 Setting entry with key 'array_int' to value '2147483648'
 Value 2147483648.000000 for parameter 'array_int' out of range [0 - 2.14748e+09]
 Error 'array_int=2147483648'
+flags=5
 
 Testing av_opt_set_from_string()
 Setting options string ''