diff --git a/image/decoders/icon/nsIconURI.cpp b/image/decoders/icon/nsIconURI.cpp
index 3ae683590e44..1da535a2c78b 100644
--- a/image/decoders/icon/nsIconURI.cpp
+++ b/image/decoders/icon/nsIconURI.cpp
@@ -418,8 +418,11 @@ nsMozIconURI::EqualsExceptRef(nsIURI* other, bool* result) {
 
 NS_IMETHODIMP
 nsMozIconURI::SchemeIs(const char* aScheme, bool* aEquals) {
-  MOZ_ASSERT(aScheme);
   MOZ_ASSERT(aEquals, "null pointer");
+  if (!aScheme) {
+    *aEquals = false;
+    return NS_OK;
+  }
 
   *aEquals = PL_strcasecmp("moz-icon", aScheme) ? false : true;
   return NS_OK;
diff --git a/modules/libjar/nsJARURI.cpp b/modules/libjar/nsJARURI.cpp
index 5b8ebc89adc2..628cec45433b 100644
--- a/modules/libjar/nsJARURI.cpp
+++ b/modules/libjar/nsJARURI.cpp
@@ -475,8 +475,11 @@ nsJARURI::EqualsExceptRef(nsIURI *other, bool *result) {
 
 NS_IMETHODIMP
 nsJARURI::SchemeIs(const char *i_Scheme, bool *o_Equals) {
-  MOZ_ASSERT(i_Scheme);
   MOZ_ASSERT(o_Equals);
+  if (!i_Scheme) {
+    *o_Equals = false;
+    return NS_OK;
+  }
 
   *o_Equals = PL_strcasecmp("jar", i_Scheme) ? false : true;
   return NS_OK;
diff --git a/netwerk/base/nsSimpleURI.cpp b/netwerk/base/nsSimpleURI.cpp
index 72d0995c7d24..605fec3043fa 100644
--- a/netwerk/base/nsSimpleURI.cpp
+++ b/netwerk/base/nsSimpleURI.cpp
@@ -523,8 +523,11 @@ bool nsSimpleURI::EqualsInternal(nsSimpleURI *otherUri,
 
 NS_IMETHODIMP
 nsSimpleURI::SchemeIs(const char *i_Scheme, bool *o_Equals) {
-  MOZ_ASSERT(i_Scheme);
   MOZ_ASSERT(o_Equals, "null pointer");
+  if (!i_Scheme) {
+    *o_Equals = false;
+    return NS_OK;
+  }
 
   const char *this_scheme = mScheme.get();
 
diff --git a/netwerk/base/nsStandardURL.cpp b/netwerk/base/nsStandardURL.cpp
index b5c371b2aa01..5a0222e0b957 100644
--- a/netwerk/base/nsStandardURL.cpp
+++ b/netwerk/base/nsStandardURL.cpp
@@ -2204,8 +2204,11 @@ nsresult nsStandardURL::EqualsInternal(
 
 NS_IMETHODIMP
 nsStandardURL::SchemeIs(const char *scheme, bool *result) {
-  MOZ_ASSERT(scheme);
   MOZ_ASSERT(result, "null pointer");
+  if (!scheme) {
+    *result = false;
+    return NS_OK;
+  }
 
   *result = SegmentIs(mScheme, scheme);
   return NS_OK;
diff --git a/netwerk/test/unit/test_URIs.js b/netwerk/test/unit/test_URIs.js
index 52169a64628a..2384cea110db 100644
--- a/netwerk/test/unit/test_URIs.js
+++ b/netwerk/test/unit/test_URIs.js
@@ -610,12 +610,27 @@ function check_space_escaping()
   uri = gIoService.newURI("http://example.com/test%20path#test%20path");
 }
 
+function check_schemeIsNull()
+{
+  let uri = gIoService.newURI("data:text/plain,aaa");
+  Assert.ok(!uri.schemeIs(null));
+  uri = gIoService.newURI("http://example.com");
+  Assert.ok(!uri.schemeIs(null));
+  uri = gIoService.newURI("dummyscheme://example.com");
+  Assert.ok(!uri.schemeIs(null));
+  uri = gIoService.newURI("jar:resource://gre/chrome.toolkit.jar!/");
+  Assert.ok(!uri.schemeIs(null));
+  uri = gIoService.newURI("moz-icon://.unknown?size=32");
+  Assert.ok(!uri.schemeIs(null));
+}
+
 // TEST MAIN FUNCTION
 // ------------------
 function run_test()
 {
   check_nested_mutations();
   check_space_escaping();
+  check_schemeIsNull();
 
   // UTF-8 check - From bug 622981
   // ASCII