diff --git a/src/libraries/System.Private.CoreLib/src/System/Span.cs b/src/libraries/System.Private.CoreLib/src/System/Span.cs
index a5580b2472d078..b512a53a632a7b 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Span.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Span.cs
@@ -284,18 +284,11 @@ public void Fill(T value)
         {
             if (Unsafe.SizeOf<T>() == 1)
             {
-#if MONO
-                // Mono runtime's implementation of initblk performs a null check on the address.
-                // We'll perform a length check here to avoid passing a null address in the empty span case.
-                if (_length != 0)
-#endif
-                {
-                    // Special-case single-byte types like byte / sbyte / bool.
-                    // The runtime eventually calls memset, which can efficiently support large buffers.
-                    // We don't need to check IsReferenceOrContainsReferences because no references
-                    // can ever be stored in types this small.
-                    Unsafe.InitBlockUnaligned(ref Unsafe.As<T, byte>(ref _pointer.Value), Unsafe.As<T, byte>(ref value), (uint)_length);
-                }
+                // Special-case single-byte types like byte / sbyte / bool.
+                // The runtime eventually calls memset, which can efficiently support large buffers.
+                // We don't need to check IsReferenceOrContainsReferences because no references
+                // can ever be stored in types this small.
+                Unsafe.InitBlockUnaligned(ref Unsafe.As<T, byte>(ref _pointer.Value), Unsafe.As<T, byte>(ref value), (uint)_length);
             }
             else
             {
diff --git a/src/mono/mono/mini/interp/interp.c b/src/mono/mono/mini/interp/interp.c
index 56d4134d91a700..040e1169c77f43 100644
--- a/src/mono/mono/mini/interp/interp.c
+++ b/src/mono/mono/mini/interp/interp.c
@@ -6804,18 +6804,20 @@ MINT_IN_CASE(MINT_BRTRUE_I8_SP) ZEROP_SP(gint64, !=); MINT_IN_BREAK;
 		MINT_IN_CASE(MINT_CPBLK) {
 			gpointer dest = LOCAL_VAR (ip [1], gpointer);
 			gpointer src = LOCAL_VAR (ip [2], gpointer);
-			if (!dest || !src)
+			guint32 size = LOCAL_VAR (ip [3], guint32);
+			if (size && (!dest || !src))
 				THROW_EX (mono_get_exception_null_reference(), ip);
-			/* FIXME: value and size may be int64... */
-			memcpy (dest, src, LOCAL_VAR (ip [3], gint32));
+			else
+				memcpy (dest, src, size);
 			ip += 4;
 			MINT_IN_BREAK;
 		}
 		MINT_IN_CASE(MINT_INITBLK) {
 			gpointer dest = LOCAL_VAR (ip [1], gpointer);
-			NULL_CHECK (dest);
-			/* FIXME: value and size may be int64... */
-			memset (dest, LOCAL_VAR (ip [2], gint32), LOCAL_VAR (ip [3], gint32));
+			guint32 size = LOCAL_VAR (ip [3], guint32);
+			if (size)
+				NULL_CHECK (dest);
+			memset (dest, LOCAL_VAR (ip [2], gint32), size);
 			ip += 4;
 			MINT_IN_BREAK;
 		}