diff --git a/src/ExtismValType.php b/src/ExtismValType.php
index 0a78b0f..f3d5084 100644
--- a/src/ExtismValType.php
+++ b/src/ExtismValType.php
@@ -6,7 +6,7 @@ class ExtismValType
 {
     public const I32 = 0;
     public const I64 = 1;
-    public const PTR = I64;
+    public const PTR = ExtismValType::I64;
     public const F32 = 2;
     public const F64 = 3;
     public const V128 = 4;
diff --git a/src/FunctionCallException.php b/src/FunctionCallException.php
new file mode 100644
index 0000000..ff348a7
--- /dev/null
+++ b/src/FunctionCallException.php
@@ -0,0 +1,17 @@
+<?php
+
+namespace Extism;
+
+class FunctionCallException extends \Exception
+{
+    public string $error;
+    public string $functionName;
+
+    public function __construct(string $message, string $error, string $functionName)
+    {
+        parent::__construct($message);
+
+        $this->error = $error;
+        $this->functionName = $functionName;
+    }
+}
\ No newline at end of file
diff --git a/src/Internal/LibExtism.php b/src/Internal/LibExtism.php
index c145469..f0bda39 100644
--- a/src/Internal/LibExtism.php
+++ b/src/Internal/LibExtism.php
@@ -52,7 +52,7 @@ private function soname()
             case "Windows NT":
                 return "extism.dll";
             default:
-                throw new \Exception("Extism: unsupported platform " . $platform);
+                throw new \RuntimeException("Extism: unsupported platform " . $platform);
         }
     }
 
diff --git a/src/Manifest/PathWasmSource.php b/src/Manifest/PathWasmSource.php
index 72b243d..1306ab4 100644
--- a/src/Manifest/PathWasmSource.php
+++ b/src/Manifest/PathWasmSource.php
@@ -19,7 +19,7 @@ public function __construct($path, $name = null, $hash = null)
         $this->path = realpath($path);
 
         if (!$this->path) {
-            throw new \Exception("Path not found: '" . $path . "'");
+            throw new \Extism\PluginLoadException("Path not found: '" . $path . "'");
         }
 
         $this->name = $name ?? pathinfo($path, PATHINFO_FILENAME);
diff --git a/src/Plugin.php b/src/Plugin.php
index cb64314..4c88394 100644
--- a/src/Plugin.php
+++ b/src/Plugin.php
@@ -57,7 +57,7 @@ public function __construct(Manifest $manifest, bool $with_wasi = false, array $
         if (\FFI::isNull($errPtr) == false) {
             $error = \FFI::string($errPtr);
             $this->lib->extism_plugin_new_error_free($errPtr);
-            throw new \Exception("Extism: unable to load plugin: " . $error);
+            throw new \Extism\PluginLoadException("Extism: unable to load plugin: " . $error);
         }
 
         $this->handle = $handle;
@@ -93,13 +93,17 @@ public function functionExists(string $name)
      */
     public function call(string $name, string $input = null): string
     {
+        if ($input == null) {
+            $input = "";
+        }
+
         $rc = $this->lib->extism_plugin_call($this->handle, $name, $input, strlen($input));
 
         $msg = "code = " . $rc;
         $err = $this->lib->extism_error($this->handle);
         if ($err) {
             $msg = $msg . ", error = " . $err;
-            throw new \Exception("Extism: call to '" . $name . "' failed with " . $msg);
+            throw new \Extism\FunctionCallException("Extism: call to '" . $name . "' failed with " . $msg, $err, $name);
         }
 
         return $this->lib->extism_plugin_output_data($this->handle);
diff --git a/src/PluginLoadException.php b/src/PluginLoadException.php
new file mode 100644
index 0000000..e895a11
--- /dev/null
+++ b/src/PluginLoadException.php
@@ -0,0 +1,11 @@
+<?php
+
+namespace Extism;
+
+class PluginLoadException extends \Exception
+{
+    public function __construct(string $message)
+    {
+        parent::__construct($message);
+    }
+}
\ No newline at end of file