diff --git a/src/util/rbs_allocator.c b/src/util/rbs_allocator.c
index 2a3f17076..51fee5d8d 100644
--- a/src/util/rbs_allocator.c
+++ b/src/util/rbs_allocator.c
@@ -39,15 +39,24 @@ typedef struct rbs_allocator_page {
     size_t used;
 } rbs_allocator_page_t;
 
+// Normalize a raw page size to a safe value for payload_size subtraction.
+// Falls back to 4096 when the raw value is <= 0 or smaller than the page
+// header struct, preventing underflow in rbs_allocator_init.
+// Internal API, intentionally omitted from the public header.
+size_t rbs_allocator_normalize_page_size(long raw) {
+    if (raw <= 0) return 4096;
+    size_t page_size = (size_t) raw;
+    if (page_size <= sizeof(rbs_allocator_page_t)) return 4096;
+    return page_size;
+}
+
 static size_t get_system_page_size(void) {
 #ifdef _WIN32
     SYSTEM_INFO si;
     GetSystemInfo(&si);
-    return si.dwPageSize;
+    return rbs_allocator_normalize_page_size((long) si.dwPageSize);
 #else
-    long sz = sysconf(_SC_PAGESIZE);
-    if (sz == -1) return 4096; // Fallback to the common 4KB page size
-    return (size_t) sz;
+    return rbs_allocator_normalize_page_size(sysconf(_SC_PAGESIZE));
 #endif
 }
 
diff --git a/wasm/rbs_wasm.c b/wasm/rbs_wasm.c
index 14697d43f..efe9a609c 100644
--- a/wasm/rbs_wasm.c
+++ b/wasm/rbs_wasm.c
@@ -399,7 +399,19 @@ __attribute__((export_name("rbs_wasm_lex"))) int rbs_wasm_lex(const char *source
  *
  * @return 1 if the sample parsed successfully, 0 otherwise.
  */
+// Internal: defined in rbs_allocator.c, not declared in the public header.
+extern size_t rbs_allocator_normalize_page_size(long raw);
+
 __attribute__((export_name("rbs_wasm_selftest"))) int rbs_wasm_selftest(void) {
+    // Regression test: normalize_page_size must return a safe value
+    // (>= sizeof(rbs_allocator_page_t)) for inputs that would underflow
+    // payload_size. On WASI in a Rust cdylib, sysconf(_SC_PAGESIZE)
+    // returns 0; the normalization must catch that.
+    if (rbs_allocator_normalize_page_size(-1) != 4096) return 0;
+    if (rbs_allocator_normalize_page_size(0) != 4096) return 0;
+    if (rbs_allocator_normalize_page_size(1) != 4096) return 0;
+    if (rbs_allocator_normalize_page_size(65536) != 65536) return 0;
+
     static const char source[] =
         "class User\n"
         "  attr_reader name: String\n"