diff --git a/src/lib/strnum.c b/src/lib/strnum.c
index e97e2fc719..9479fc8e15 100644
--- a/src/lib/strnum.c
+++ b/src/lib/strnum.c
@@ -460,7 +460,9 @@ int str_to_float(const char *str, float *num_r)
 {
 	char *endp;
 	float num = strtof(str, &endp);
-	if (*endp != '\0')
+	/* strtof() performs no conversion for an empty string and leaves endp
+	   pointing at str, so the *endp check alone would accept "" as 0. */
+	if (endp == str || *endp != '\0')
 		return -1;
 
 	*num_r = num;
@@ -471,7 +473,9 @@ int str_to_double(const char *str, double *num_r)
 {
 	char *endp;
 	double num = strtod(str, &endp);
-	if (*endp != '\0')
+	/* strtod() performs no conversion for an empty string and leaves endp
+	   pointing at str, so the *endp check alone would accept "" as 0. */
+	if (endp == str || *endp != '\0')
 		return -1;
 
 	*num_r = num;
diff --git a/src/lib/test-strnum.c b/src/lib/test-strnum.c
index 5432af940d..857857fe60 100644
--- a/src/lib/test-strnum.c
+++ b/src/lib/test-strnum.c
@@ -375,8 +375,10 @@ static void test_str_to_float(void)
 		INVALID(0.1x),
 		INVALID(0.1.2),
 		INVALID(bad),
+		/* strtof()/strtod() consume nothing here, so endp stays at str */
+		{ "", -1, 0 },
 	};
-	test_begin("str_to_int");
+	test_begin("str_to_float");
 	for (i = 0; i < N_ELEMENTS(tests); ++i) {
 		float val = 1234.0;
 		double val2 = 1234.0;