diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 325e819bb..d418fff3a 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -20467,6 +20467,14 @@ class binary_reader
                 return false;
             }
 
+            // validate the type marker up front: otherwise it would only be
+            // checked when an element is read, and a container with count 0
+            // would accept any marker
+            if (JSON_HEDLEY_UNLIKELY(!is_valid_optimized_type(result.second)))
+            {
+                return optimized_type_error();
+            }
+
             get_ignore_noop();
             if (JSON_HEDLEY_UNLIKELY(current != '#'))
             {
@@ -21963,6 +21971,44 @@ class binary_reader
     std::size_t bon8_pushback_size = 0;
 
   JSON_PRIVATE_UNLESS_TESTED:
+    /*!
+    @brief check whether @a marker is a type marker that may follow '$'
+    UBJSON: Z N T F i U I l L d D H C S [ {; BJData: the numeric markers and C
+    (BJData's excluded markers are rejected separately).
+    */
+    bool is_valid_optimized_type(const char_int_type marker) const noexcept
+    {
+        switch (marker)
+        {
+            case 'i':
+            case 'U':
+            case 'I':
+            case 'l':
+            case 'L':
+            case 'd':
+            case 'D':
+            case 'C':
+                return true;
+            case 'Z':
+            case 'N':
+            case 'T':
+            case 'F':
+            case 'H':
+            case 'S':
+            case '[':
+            case '{':
+                return input_format != input_format_t::bjdata;
+            case 'B':
+            case 'u':
+            case 'm':
+            case 'M':
+            case 'h':
+                return input_format == input_format_t::bjdata;
+            default:
+                return false;
+        }
+    }
+
     /*!
     @brief whether @a marker is excluded from BJData's optimized ND-array types
     @return whether @a marker is one of 'F', 'H', 'N', 'S', 'T', 'Z', '[', '{'
diff --git a/tests/src/unit-bjdata.cpp b/tests/src/unit-bjdata.cpp
index af92b48d1..db0663d52 100644
--- a/tests/src/unit-bjdata.cpp
+++ b/tests/src/unit-bjdata.cpp
@@ -4629,11 +4629,18 @@ TEST_CASE("BJData optimized container type marker is validated for count 0")
     SECTION("invalid marker")
     {
         // 'Z', 'T', 'F', 'N', 'S', 'H', '[' and '{' are excluded in BJData
-        for (const char marker : {'x', 'E', 'Z', 'T', 'F', 'N', 'S', 'H', '[', '{'})
+        for (const char marker :
+                {'x', 'E', 'Z', 'T', 'F', 'N', 'S', 'H', '[', '{'
+                })
         {
-            for (const char open : {'[', '{'})
+            for (const char open :
+                    {'[', '{'
+                    })
             {
-                for (const std::uint8_t count : {0, 1})
+                for (const std::uint8_t count :
+                        {
+                            0, 1
+                        })
                 {
                     CAPTURE(marker)
                     CAPTURE(open)
@@ -4656,7 +4663,9 @@ TEST_CASE("BJData optimized container type marker is validated for count 0")
 
     SECTION("valid marker")
     {
-        for (const char marker : {'i', 'U', 'I', 'u', 'l', 'm', 'L', 'M', 'h', 'd', 'D', 'C'})
+        for (const char marker :
+                {'i', 'U', 'I', 'u', 'l', 'm', 'L', 'M', 'h', 'd', 'D', 'C'
+                })
         {
             CAPTURE(marker)
             const std::vector<std::uint8_t> va = {'[', '$', static_cast<std::uint8_t>(marker), '#', 'i', 0};
diff --git a/tests/src/unit-ubjson.cpp b/tests/src/unit-ubjson.cpp
index db6ac0f3a..8f6c42810 100644
--- a/tests/src/unit-ubjson.cpp
+++ b/tests/src/unit-ubjson.cpp
@@ -3354,11 +3354,18 @@ TEST_CASE("UBJSON optimized container type marker is validated for count 0")
     // the '$' type marker must be a legal marker even if no element follows
     SECTION("invalid marker")
     {
-        for (const char marker : {'x', 'E', 'B', 'u', 'm', 'M', 'h'})
+        for (const char marker :
+                {'x', 'E', 'B', 'u', 'm', 'M', 'h'
+                })
         {
-            for (const char open : {'[', '{'})
+            for (const char open :
+                    {'[', '{'
+                    })
             {
-                for (const std::uint8_t count : {0, 1})
+                for (const std::uint8_t count :
+                        {
+                            0, 1
+                        })
                 {
                     CAPTURE(marker)
                     CAPTURE(open)
@@ -3381,7 +3388,9 @@ TEST_CASE("UBJSON optimized container type marker is validated for count 0")
 
     SECTION("valid marker")
     {
-        for (const char marker : {'Z', 'N', 'T', 'F', 'i', 'U', 'I', 'l', 'L', 'd', 'D', 'H', 'C', 'S', '[', '{'})
+        for (const char marker :
+                {'Z', 'N', 'T', 'F', 'i', 'U', 'I', 'l', 'L', 'd', 'D', 'H', 'C', 'S', '[', '{'
+                })
         {
             CAPTURE(marker)
             const std::vector<std::uint8_t> va = {'[', '$', static_cast<std::uint8_t>(marker), '#', 'i', 0};