diff --git a/source/dub/dependencyresolver.d b/source/dub/dependencyresolver.d
index 3829edc..3635e58 100644
--- a/source/dub/dependencyresolver.d
+++ b/source/dub/dependencyresolver.d
@@ -86,7 +86,7 @@
 					package_names[pidx] = basepack;
 				}
 
-				configs = getSpecificConfigs(ch) ~ configs;
+				configs = getSpecificConfigs(basepack, ch) ~ configs;
 
 				// eliminate configurations from which we know that they can't satisfy
 				// the uniquely defined root dependencies (==version or ~branch style dependencies)
@@ -109,7 +109,7 @@
 				cfgs = cfgs ~ CONFIG.invalid;
 
 		logDebug("Configurations used for dependency resolution:");
-		foreach (n, i; package_indices) logDebug("  %s (%s): %s", n, i, all_configs[i]);
+		foreach (n, i; package_indices) logDebug("  %s (%s%s): %s", n, i, n in required_deps ? "" : ", optional", all_configs[i]);
 
 		auto config_indices = new size_t[all_configs.length];
 		config_indices[] = 0;
@@ -225,7 +225,7 @@
 	}
 
 	protected abstract CONFIG[] getAllConfigs(string pack);
-	protected abstract CONFIG[] getSpecificConfigs(TreeNodes nodes);
+	protected abstract CONFIG[] getSpecificConfigs(string pack, TreeNodes nodes);
 	protected abstract TreeNodes[] getChildren(TreeNode node);
 	protected abstract bool matches(CONFIGS configs, CONFIG config);
 
@@ -293,7 +293,7 @@
 			ret.data.sort!"a>b"();
 			return ret.data;
 		}
-		protected override IntConfig[] getSpecificConfigs(TreeNodes nodes) { return null; }
+		protected override IntConfig[] getSpecificConfigs(string pack, TreeNodes nodes) { return null; }
 		protected override TreeNodes[] getChildren(TreeNode node) { return m_children.get(node.pack ~ ":" ~ node.config.to!string(), null); }
 		protected override bool matches(IntConfigs configs, IntConfig config) { return configs.canFind(config); }
 	}
diff --git a/source/dub/dub.d b/source/dub/dub.d
index 403d3e4..afdbd14 100644
--- a/source/dub/dub.d
+++ b/source/dub/dub.d
@@ -1040,16 +1040,24 @@
 		if (!(m_options & UpgradeOptions.preRelease))
 			versions = versions.filter!(v => !v.isPreRelease).array ~ versions.filter!(v => v.isPreRelease).array;
 
+		// filter out invalid/unreachable dependency specs
+		versions = versions.filter!((v) {
+				bool valid = getPackage(pack, Dependency(v)) !is null;
+				if (!valid) logDiagnostic("Excluding invalid dependency specification %s %s from dependency resolution process.", pack, v);
+				return valid;
+			}).array;
+
 		if (!versions.length) logDiagnostic("Nothing found for %s", pack);
+		else logDiagnostic("Return for %s: %s", pack, versions);
 
 		auto ret = versions.map!(v => Dependency(v)).array;
 		m_packageVersions[pack] = ret;
 		return ret;
 	}
 
-	protected override Dependency[] getSpecificConfigs(TreeNodes nodes)
+	protected override Dependency[] getSpecificConfigs(string pack, TreeNodes nodes)
 	{
-		if (!nodes.configs.path.empty) return [nodes.configs];
+		if (!nodes.configs.path.empty && getPackage(pack, nodes.configs)) return [nodes.configs];
 		else return null;
 	}