diff --git a/tools/web.py b/tools/web.py
index 61fe96dd..c6e57b88 100644
--- a/tools/web.py
+++ b/tools/web.py
@@ -36,12 +36,16 @@ def generate(env):
 
     # Thread support (via SharedArrayBuffer).
     if env["threads"]:
-        env.Append(CCFLAGS=["-s", "USE_PTHREADS=1"])
-        env.Append(LINKFLAGS=["-s", "USE_PTHREADS=1"])
+        env.Append(CCFLAGS=["-sUSE_PTHREADS=1"])
+        env.Append(LINKFLAGS=["-sUSE_PTHREADS=1"])
 
     # Build as side module (shared library).
-    env.Append(CPPFLAGS=["-s", "SIDE_MODULE=1"])
-    env.Append(LINKFLAGS=["-s", "SIDE_MODULE=1"])
+    env.Append(CPPFLAGS=["-sSIDE_MODULE=1"])
+    env.Append(LINKFLAGS=["-sSIDE_MODULE=1"])
+
+    # Force wasm longjmp mode.
+    env.Append(CCFLAGS=["-sSUPPORT_LONGJMP='wasm'"])
+    env.Append(LINKFLAGS=["-sSUPPORT_LONGJMP='wasm'"])
 
     env.Append(CPPDEFINES=["WEB_ENABLED", "UNIX_ENABLED"])