diff --git a/biz.aQute.bndlib/src/aQute/bnd/build/Project.java b/biz.aQute.bndlib/src/aQute/bnd/build/Project.java
index c4338e8954..fb147c90e7 100644
--- a/biz.aQute.bndlib/src/aQute/bnd/build/Project.java
+++ b/biz.aQute.bndlib/src/aQute/bnd/build/Project.java
@@ -498,19 +498,19 @@ public File getSrc() throws Exception {
 	}
 
 	public File getSrcOutput() {
-		return getFile(getProperty(Constants.DEFAULT_PROP_BIN_DIR));
+		return getSingleFile(Constants.DEFAULT_PROP_BIN_DIR);
 	}
 
 	public File getTestSrc() {
-		return getFile(getProperty(Constants.DEFAULT_PROP_TESTSRC_DIR));
+		return getSingleFile(Constants.DEFAULT_PROP_TESTSRC_DIR);
 	}
 
 	public File getTestOutput() {
-		return getFile(getProperty(Constants.DEFAULT_PROP_TESTBIN_DIR));
+		return getSingleFile(Constants.DEFAULT_PROP_TESTBIN_DIR);
 	}
 
 	public File getTargetDir() {
-		return getFile(getProperty(Constants.DEFAULT_PROP_TARGET_DIR));
+		return getSingleFile(Constants.DEFAULT_PROP_TARGET_DIR);
 	}
 
 	private void traverse(Set<Project> dependencies, Project dependent, Set<Project> visited) throws Exception {
@@ -524,6 +524,17 @@ private void traverse(Set<Project> dependencies, Project dependent, Set<Project>
 		dependents.add(dependent);
 	}
 
+	private File getSingleFile(String key) {
+		String value = getProperty(key);
+		if (value == null) {
+			error("project.%s expected value for property %s but got null", key, key);
+			value = key;
+		} else if (value.indexOf(',') >= 0) {
+			error("project.%s expected one file path for property %s but got multiple: %s", key, key, value);
+		}
+		return getFile(value);
+	}
+
 	/**
 	 * Iterate over the entries and place the projects on the projects list and
 	 * all the files of the entries on the resultpath.
@@ -1921,8 +1932,7 @@ public File[] buildLocal(boolean underTest) throws Exception {
 							if (lastModified < jar.lastModified()) {
 								lastModified = jar.lastModified();
 							}
-							Supplier<org.osgi.resource.Resource> indexer = ResourceBuilder.memoize(jar,
-								file.toURI(),
+							Supplier<org.osgi.resource.Resource> indexer = ResourceBuilder.memoize(jar, file.toURI(),
 								getName());
 							if (indexer != null) {
 								resourceBuilders.add(indexer);
@@ -1987,7 +1997,6 @@ public File[] buildLocal(boolean underTest) throws Exception {
 					}
 				}
 
-
 				boolean bfsWrite = !bfs.exists() || (lastModified > bfs.lastModified());
 				if (buildfiles != null) {
 					Set<File> removed = Create.set(buildfiles);