diff --git a/include/fast_float/ascii_number.h b/include/fast_float/ascii_number.h
index f9680ec..21fbac4 100644
--- a/include/fast_float/ascii_number.h
+++ b/include/fast_float/ascii_number.h
@@ -280,7 +280,7 @@ parsed_number_string_t<UC> parse_number_string(UC const *p, UC const * pend, par
   answer.too_many_digits = false;
   answer.negative = (*p == UC('-'));
 #ifdef FASTFLOAT_ALLOWS_LEADING_PLUS // disabled by default
-  if ((*p == UC('-')) || (*p == UC('+'))) {
+  if ((*p == UC('-')) || (fmt != chars_format::json && *p == UC('+'))) {
 #else
   if (*p == UC('-')) { // C++17 20.19.3.(7.1) explicitly forbids '+' sign here
 #endif
diff --git a/tests/json_fmt.cpp b/tests/json_fmt.cpp
index 0c1c365..b1c3977 100644
--- a/tests/json_fmt.cpp
+++ b/tests/json_fmt.cpp
@@ -3,13 +3,16 @@
 #include <iostream>
 #include <vector>
 
+// test that this option is ignored
+#define FASTFLOAT_ALLOWS_LEADING_PLUS
+
 #include "fast_float/fast_float.h"
 
 int main()
 {
   const std::vector<double> expected{ -0.2, 0.02, 0.002, 1., 0. };
   const std::vector<std::string> accept{ "-0.2", "0.02", "0.002", "1e+0000", "0e-2" };
-  const std::vector<std::string> reject{ "-.2", "00.02", "0.e+1", "00.e+1", ".25"};
+  const std::vector<std::string> reject{ "-.2", "00.02", "0.e+1", "00.e+1", ".25", "+0.25"};
   const auto fmt = fast_float::chars_format::json;
 
   for (std::size_t i = 0; i < accept.size(); ++i)