diff --git a/bootstrap.sh b/bootstrap.sh
index 11791a8..47e946d 100755
--- a/bootstrap.sh
+++ b/bootstrap.sh
@@ -5,7 +5,7 @@
 # To do this you need to vanilla composer package manager installed on your
 # system. You can install composer from Nixpkgs as follows:
 #
-# nix-env -f '<nixpkgs>' -iA php phpPackages.composer
+# nix-env -f '<nixpkgs>' -iA php phpPackages.composer nix-prefetch-scripts
 
 composer install
 php bin/composer2nix --composer-env src/Composer2Nix/composer-env.nix --no-copy-composer-env --executable
diff --git a/src/Composer2Nix/Sources/GitSource.php b/src/Composer2Nix/Sources/GitSource.php
index 0a32d76..8a9a76c 100644
--- a/src/Composer2Nix/Sources/GitSource.php
+++ b/src/Composer2Nix/Sources/GitSource.php
@@ -1,5 +1,6 @@
 <?php
 namespace Composer2Nix\Sources;
+use Exception;
 use PNDP\NixGenerator;
 use PNDP\AST\NixExpression;
 use PNDP\AST\NixFunInvocation;
@@ -30,7 +31,7 @@ class GitSource extends Source
 	{
 		$outputStr = shell_exec('nix-prefetch-git "'.$this->sourceObj['url'].'" '.$this->sourceObj["reference"]);
 
-		if($outputStr === false)
+		if($outputStr === false || $outputStr === null)
 			throw new Exception("Error while invoking nix-prefetch-git");
 		else
 		{
diff --git a/src/Composer2Nix/Sources/HgSource.php b/src/Composer2Nix/Sources/HgSource.php
index 9c2c210..e09b220 100644
--- a/src/Composer2Nix/Sources/HgSource.php
+++ b/src/Composer2Nix/Sources/HgSource.php
@@ -1,5 +1,6 @@
 <?php
 namespace Composer2Nix\Sources;
+use Exception;
 use PNDP\NixGenerator;
 use PNDP\AST\NixExpression;
 use PNDP\AST\NixFunInvocation;
diff --git a/src/Composer2Nix/Sources/SVNSource.php b/src/Composer2Nix/Sources/SVNSource.php
index de9c582..29f2d95 100644
--- a/src/Composer2Nix/Sources/SVNSource.php
+++ b/src/Composer2Nix/Sources/SVNSource.php
@@ -1,5 +1,6 @@
 <?php
 namespace Composer2Nix\Sources;
+use Exception;
 use PNDP\NixGenerator;
 use PNDP\AST\NixExpression;
 use PNDP\AST\NixFunInvocation;
diff --git a/src/Composer2Nix/Sources/Source.php b/src/Composer2Nix/Sources/Source.php
index 28e2728..0248ef6 100644
--- a/src/Composer2Nix/Sources/Source.php
+++ b/src/Composer2Nix/Sources/Source.php
@@ -1,7 +1,7 @@
 <?php
 namespace Composer2Nix\Sources;
-use PNDP\AST\NixASTNode;
 use Exception;
+use PNDP\AST\NixASTNode;
 
 /**
  * Represents a dependency of a composer package that can be converted into a
diff --git a/src/Composer2Nix/Sources/ZipSource.php b/src/Composer2Nix/Sources/ZipSource.php
index 4a843f5..c8b3450 100644
--- a/src/Composer2Nix/Sources/ZipSource.php
+++ b/src/Composer2Nix/Sources/ZipSource.php
@@ -1,5 +1,6 @@
 <?php
 namespace Composer2Nix\Sources;
+use Exception;
 use PNDP\NixGenerator;
 use PNDP\AST\NixExpression;
 use PNDP\AST\NixFile;
diff --git a/tests/dependencies/php-packages.nix b/tests/dependencies/php-packages.nix
index 36a2735..ccfccf0 100644
--- a/tests/dependencies/php-packages.nix
+++ b/tests/dependencies/php-packages.nix
@@ -598,7 +598,7 @@ let
         name = "svanderburg-php-sblayout-83d6d110494cdceb13da48a1acfa0ae989ba9ecd";
         url = "https://github.com/svanderburg/php-sblayout.git";
         rev = "83d6d110494cdceb13da48a1acfa0ae989ba9ecd";
-        sha256 = null;
+        sha256 = "1nxvppsxn6i4gczzmjlr50p94yw8v50s6l87qvpiyhh5162dyr3n";
       };
     };
     "symfony/cache" = {