diff --git a/src/meta/adp_ongakukan.c b/src/meta/adp_ongakukan.c
index 1284c836..552a9d22 100644
--- a/src/meta/adp_ongakukan.c
+++ b/src/meta/adp_ongakukan.c
@@ -9,8 +9,8 @@ VGMSTREAM* init_vgmstream_adp_ongakukan(STREAMFILE* sf) {
     bool sound_is_adpcm = false;
     int32_t fmt_size, fmt_offset;
     int32_t sample_rate, data_size;
-    int16_t channels;
-
+    int16_t channels;
+    int32_t expected_size, pcm_size, diff, fact_offset;
 
     /* checks */
     if (!is_id32be(0x00, sf, "RIFF"))
@@ -24,21 +24,22 @@ VGMSTREAM* init_vgmstream_adp_ongakukan(STREAMFILE* sf) {
     data_size = get_streamfile_size(sf) - start_offset;
 
     /* RIFF size seem to match original PCM .wav, while encoded .adp data equals or is slightly smaller that that */
-    uint32_t expected_size = (read_u32le(0x04, sf) - 0x24);
-    uint32_t pcm_size = data_size * 2 * sizeof(short); // * channels
-    if (pcm_size > expected_size)
+    expected_size = (read_u32le(0x04, sf) - 0x24);
+    pcm_size = data_size * 2 * sizeof(short); // * channels
+    diff = expected_size - pcm_size;
+    if (diff < 0 || diff > 2)
         return NULL;
 
     if (!is_id32be(0x08, sf, "WAVE"))
         return NULL;
-    if (!is_id32be(0x0c, sf, "fmt ")) 
-        return NULL;
+    if (!is_id32be(0x0c, sf, "fmt "))
+        return NULL;
 
     fmt_size = read_s32le(0x10, sf);
     /* depending on the adp, fmt_size alternates between 0x10 and 0x12 */
     if (fmt_size < 0x10 || fmt_size > 0x12)
         goto fail;
-    fmt_offset = 0x14;
+    fmt_offset = 0x14;
 
     if (read_s16le(fmt_offset + 0x00, sf) != 0x0001) /* PCM format */
         goto fail;
@@ -49,11 +50,17 @@ VGMSTREAM* init_vgmstream_adp_ongakukan(STREAMFILE* sf) {
     if (read_s16le(fmt_offset + 0x0e, sf) != 16) /* PCM bit depth */
         goto fail;
     /* rest of fmt header is the usual header for 16-bit PCM wav files: bitrate, block size, and the like (see riff.c) */
-    /* if fmt_size == 0x12 there may be is an additional s16 field that's always zero, but not always. */
-
-    /* next chunk is at fixed offset, regardless of fmt_size (fmt_size 0x12 with "data" at 0x24 is possible).
-     * "data" has chunk size (does not match ADP size but original WAV) and "fact" chunk size 0x04 cut off) */
-    if (!is_id32be(0x24, sf, "data") && !is_id32be(0x26, sf, "fact"))
+    /* if fmt_size == 0x12 there is an additional s16 field that may or may not be zero. */
+
+    /* depending on the adp, "fact" chunk is "movable" as we'll see later. */
+    fact_offset = fmt_offset + fmt_size;
+    if (fact_offset < 0x24 || fact_offset > 0x28)
+        goto fail;
+
+    /* for next chunk, if "data" then it's at fixed offset (0x24), regardless of fmt_size (fmt_size 0x12 with "data" at 0x24 is possible).
+     * if "fact" however, offset goes AFTER fmt_size (mostly 0x26 and rarely 0x24).
+     * "data" has chunk size (does not match ADP size but original WAV) and "fact" chunk size is always 0x04 (whether cut off or otherwise). */
+    if (!is_id32be(0x24, sf, "data") && !is_id32be(fact_offset, sf, "fact"))
         goto fail;
 
     /* Ongagukan games using this format just read it by checking "ADP" extension in a provided file name,