diff --git a/packages/exec/src/toolrunner.ts b/packages/exec/src/toolrunner.ts
index 940d9a1a40..87d0bbe10e 100644
--- a/packages/exec/src/toolrunner.ts
+++ b/packages/exec/src/toolrunner.ts
@@ -84,7 +84,7 @@ export class ToolRunner extends events.EventEmitter {
     data: Buffer,
     strBuffer: string,
     onLine: (line: string) => void
-  ): void {
+  ): string {
     try {
       let s = strBuffer + data.toString()
       let n = s.indexOf(os.EOL)
@@ -98,10 +98,12 @@ export class ToolRunner extends events.EventEmitter {
         n = s.indexOf(os.EOL)
       }
 
-      strBuffer = s
+      return s
     } catch (err) {
       // streaming lines to console is best effort.  Don't fail a build.
       this._debug(`error processing line. Failed with error ${err}`)
+
+      return ''
     }
   }
 
@@ -444,7 +446,7 @@ export class ToolRunner extends events.EventEmitter {
         this._getSpawnOptions(this.options, fileName)
       )
 
-      const stdbuffer = ''
+      let stdbuffer = ''
       if (cp.stdout) {
         cp.stdout.on('data', (data: Buffer) => {
           if (this.options.listeners && this.options.listeners.stdout) {
@@ -455,15 +457,19 @@ export class ToolRunner extends events.EventEmitter {
             optionsNonNull.outStream.write(data)
           }
 
-          this._processLineBuffer(data, stdbuffer, (line: string) => {
-            if (this.options.listeners && this.options.listeners.stdline) {
-              this.options.listeners.stdline(line)
+          stdbuffer = this._processLineBuffer(
+            data,
+            stdbuffer,
+            (line: string) => {
+              if (this.options.listeners && this.options.listeners.stdline) {
+                this.options.listeners.stdline(line)
+              }
             }
-          })
+          )
         })
       }
 
-      const errbuffer = ''
+      let errbuffer = ''
       if (cp.stderr) {
         cp.stderr.on('data', (data: Buffer) => {
           state.processStderr = true
@@ -482,11 +488,15 @@ export class ToolRunner extends events.EventEmitter {
             s.write(data)
           }
 
-          this._processLineBuffer(data, errbuffer, (line: string) => {
-            if (this.options.listeners && this.options.listeners.errline) {
-              this.options.listeners.errline(line)
+          errbuffer = this._processLineBuffer(
+            data,
+            errbuffer,
+            (line: string) => {
+              if (this.options.listeners && this.options.listeners.errline) {
+                this.options.listeners.errline(line)
+              }
             }
-          })
+          )
         })
       }